Space reclamation system and method for use in connection with tape logging system

ABSTRACT

A digital data processing system comprises a host information generating device, a mass storage subsystem, and a back-up information storage subsystem. The host information generating device generates information and provides it to the mass storage subsystem for storage. The mass storage subsystem receives the generated information from the host information generating device and transfers the generated information to the storage element for storage, and further transfers the generated information to the back-up information storage subsystem. The back-up information storage subsystem receives and stores the generated information from the mass storage subsystem&#39;s control element. The back-up information storage subsystem includes a filter/buffer module, a tape log module and a reconstruction module. The filter/buffer module filters and buffers the information received from the mass storage subsystem and provides the buffered information to the tape log module for storage. The tape log module stores the information received from the filter/buffer module in logging fashion on tape cartridges. The filter/buffer module filters the information received from the mass storage subsystem so as to reduce the amount of information to be logged, so that, if the host changes the information while it is being buffered, the filter/buffer module will provide only the most recent information to the tape log module for storage. If a failure occurs in the mass storage subsystem, the reconstruction module can reconstruct the information that was on the failed device using the stored information from the tape log module and the buffered information.

FIELD OF THE INVENTION

The invention relates generally to digital data storage subsystems foruse in storing information from, for example, digital computers. Theinvention more particularly relates to storage subsystems which may beused as back-up stores for one or more digital computer systems, andwhich further may be remotely-located from one or more of the digitalcomputer systems so as to ensure that catastrophic failure which mayoccur at the sites of the respective digital computer systems do notresult in unavailability of the information stored thereon.

BACKGROUND OF THE INVENTION

Digital computer systems are used in a number of applications in whichvirtually continuous availability of data is important to the operationof businesses or other entities using the systems. Generally, computercenters will periodically produce back-up copies of data on theirvarious digital computer systems. Such back-up copies are usually notmaintained on a continuous basis, but instead at particular points intime, often at night, and in any case represent the data at theparticular points in time at which the back-up copies are generated.Accordingly, if a failure occurs between back-ups, data which has beenreceived and processed by the digital computer systems since the lastback-up copy was produced, may be lost.

Typically, such back-up copies will be maintained by the computercenters at their respective sites so that they may be used in the eventof a failure, although some off-site archival back-ups may bemaintained. Significant additional problems arise in the case of, forexample, catastrophic events that can occur, such as may result from,for example, fire, flood or other natural disasters, intentionaltampering or sabotage and the like, which may result in unintentional orintentional damage to an entire site or some significant portionthereof, since some or all of the back-up copies may also be damaged andthe data contained thereon may be unavailable.

SUMMARY OF THE INVENTION

The invention provides a new and improved digital data storage subsystemwhich provides secure remote mirrored storage of digital data for one ormore digital data processing systems.

In brief summary, the invention provides a space reclamation subsystemfor copying valid ones of a plurality of data item updates seriallyrecorded on a source storage medium, such as a magnetic tape medium,onto a target storage medium. Each data item update has a data itemidentifier which is one of a set of data item identifier values. Thespace reclamation subsystem comprises a storage medium directory, a dataitem identifier flag set, a valid data item update identifier, and avalid data item update transfer control. The storage medium directoryincludes a series of directory entries each identifying, for acorresponding one of the series of data item updates recorded on thesource storage medium, the data item identifier associated with thecorresponding one of the series of data item updates. The data itemidentifier flag setcomprises a plurality of data item identifier flags,each associated with a data item identifier, each flag having a validcondition indicating that the source storage medium has a valid dataitem update associated with the respective data item identifier flag'sassociated data item identifier stored thereon, and at least one othercondition. The valid data item update identifier uses the storage mediumdirectory and the data item identifier flag set to identify a set ofvalid data item updates on the source storage medium. In that operation,the valid data item update identifier scans the directory entries in thedirectory in reverse in reverse order and, for each directory entry,determines whether the data item identifier flag associated with thedata item identifier contained in the directory entry indicates that thesource cartridge has a valid data item update associated with therespective flag's associated data item identifier stored thereon and, ifso, determines that the data item update associated with the directoryentry is a valid data item update. The valid data item update transfercontrol provides valid ones of the data item updates as identified bythe valid data item update identifier from the source storage medium forstorage on the target storage medium.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is pointed out with particularity in the appended claims.The above and further advantages of this invention may be betterunderstood by referring to the following description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a functional block diagram of a system including a remote dataprotection facility constructed in accordance with the invention;

FIG. 2 is a functional block diagram of one embodiment of a digital dataprocessing system which the remote data protection facility;

FIG. 3 is a diagram which is useful in understanding the operation ofthe digital data processing system depicted in FIG. 2;

FIG. 4 is a functional block diagram of an input module useful in theremote data protection facility depicted in FIG. 1;

FIG. 5 is a functional block diagram of a filter/buffer module which isuseful in the remote data protection facility depicted in FIG. 1;

FIG. 6 is a functional block diagram of a tape log module useful in theremote data protection facility depicted in FIG. 1;

FIG. 7 is a functional block diagram of an output module useful in theremote data protection facility depicted in FIG. 1;

FIGS. 8 and 9 are a flow charts detailing operations performed by thefilter/buffer module's control module in controlling the filter/buffermodule depicted in FIG. 5; and

FIGS. 10 and 11 are flow charts detailing operations performed by thetape log module's tape log control module in controlling the tape logmodule depicted in FIG. 6; and

FIG. 12 is a flow chart detailing operations performed by thereconstruction module 53 depicted in FIG. 1.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

I. General

FIG. 1 is a functional block diagram of a remote data protectionfacility 5 constructed in accordance with the invention. With referenceto FIG. 1, the remote data protection facility 5 is shown connected toone or more digital data processing systems 10(0) through 10(N)(generally identified by reference numeral 10(n)) over one or morecommunication links 12(0) through 12(N) (generally identified byreference numeral 12(n)). Each digital data processing system 10(n)includes one or more host computers generally identified by referencenumeral 11(n) and an associated local mass storage subsystem generallyidentified by reference numeral 14(n). The host computer 11(n) maycomprise, for example, a mainframe computer system, a personal computer,workstation, or the like which can be connected directly or indirectlyto the respective mass storage subsystem 14(n). Each host computer 11(n)may initiate an access operation in connection with its associated localmass storage subsystem 14(n) to perform a retrieval operation, in whichthe local computer 13(n) initiates retrieval of computer programs anddigital data (which will generally be referred to herein as"information" or "data") from the local mass storage subsystem 14(n) foruse by the host computer 11(n) in its processing operations. Inaddition, each host computer 11(n) may initiate storage of processeddata in the mass storage subsystem 14(n). Generally, retrievaloperations and storage operations in connection with the mass storagesubsystems 14(n) will collectively be referred to as "accessoperations."

The mass storage subsystems 14(n) in one embodiment are generallysimilar to the digital data storage subsystem described in U. S. Pat.No. 5,206,939, entitled System And Method For Disk Mapping And DataRetrieval, issued Apr. 27, 1993 to Moshe Yanai, et al (hereinafter, "the'939 patent"), U.S. patent application Ser. No. 07/893,509 filed Jun. 4,1992, in the name of Moshe Yanai, et al., entitled "System And MethodFor Dynamically Controlling Cache Management" now U.S. Pat. No.5,381,539, issued Jan. 19, 1995, and U.S. patent application Ser. No.08/523,304, filed Sep. 5, 1995, in the name of Natan Vishlitzky, et al.,and entitled "Cache Management System Using Time Stamping ForReplacement Queue" (hereafter "Vishlitzky cache managementapplication"), now U.S. Pat. No. 5,592,432 issued Jan. 7, 1997, and U.S.patent application Ser. No. 08/619,931, filed Mar. 18, 1996, in the nameof Natan Vishlitzky, et al., and entitled "System And Method For CachingInformation In A Digital Data Storage Subsystem," all of which areassigned to the assignee of the present invention and incorporatedherein by reference.

The remote data protection facility 5 provides "mirrored" back-upprotection for data stored in the mass storage subsystems 14(n) of thevarious digital data processing systems 10(n), to protect againstnumerous types of failures, including, for example, catastrophicfailures at the sites of the respective mass storage subsystems 14(n).Such catastrophic failures may result from numerous types of events atthe respective sites of the mass storage subsystems 14(n), including,for example, fire, flood or other natural disasters, intentionaltampering or sabotage, and the like, which may result in unintentionalor intentional damage to a mass storage subsystem 14(n) and/or its siteand consequent loss of availability of the data stored in the respectivemass storage subsystem 14(n). The remote data protection facility 5 willpreferably be located geographically remotely from the sites of thedigital data processing systems 11(n) sufficiently far to ensure that,if a catastrophic failure occurs at the digital data processing system,the remote data protection facility will survive. It will be appreciatedthat remote data protection facilities may be provided at a number ofsites that are distributed geographically, and a digital data processingsystem 11(n) may be connected to remote data protection facilities atone or more of the sites. In addition, the remote data protectionfacility 5 will also protect against digital data processing systemfailures which are less than catastrophic, such as, for example, failureof some or all of the elements of the mass storage subsystems 14(n) asdescribed below, for reasons that are not limited to catastrophiccauses.

Each of the mass storage subsystems 14(n) may transfer information tothe remote data protection facility 5 over the respective communicationlink 12(n) for protected mirrored storage at the remote data protectionfacility site remote from the respective digital data processing system10(n). Furthermore, each mass storage subsystem 14(n) may transmitcontrol commands to the remote data protection facility 5 to controlcertain operations of the remote data protection facility 5. Inaddition, if the digital data processing system 10(n) requiresinformation stored on the remote data protection facility 5 forprocessing, which may be a result of an earlier catastrophic failure atthe site of the digital data processing system 10(n), failure tomaintain suitable information data protection locally at the digitaldata processing system 10(n), or the like, the digital data processingsystem 10(n) (in particular its mass storage subsystem 14(n)) mayretrieve the information that was previously stored on the remote dataprotection facility 5 for use in its subsequent processing. Furthermore,if, for example, a digital data processing system 10(n) is unavailabledue to, for example, a catastrophe at its site, another digital dataprocessing system 10(n') (n'≠n) may retrieve information from the remotedata protection facility 5 which was previously stored by the digitaldata processing system 10(n) for use in its processing, which may assistin ensuring that the information is continually available for processingeven if the digital data processing system 10(n) is not available,thereby ensuring that the information will be available to at least oneof the digital data processing systems 10(n).

The communication links 12(n) interconnecting the respective digitaldata processing systems 10(n), on the one hand, and the remote dataprotection facility 5, on the other hand, are preferably high-speed datacommunications links, such as may be used in connection with computernetworks, including, for example, optical fibers, high-speed telephonelines, and the like. The information transferred over the communicationlinks 12(n) is preferably compressed, using any convenient compressionmechanism, and some or all of the information may be encrypted toprotect against improper eavesdropping or dissemination duringcommunication over the communication links. If the remote dataprotection facility 5 is to be used in connection with information whichbelongs to multiple owners, each of the owners of the information mayuse its own encryption mechanism (such as its own encryption algorithmor its own encryption key); as will be clear from the followingdescription, the remote data protection facility 5 may store informationin encrypted or unencrypted form, but will preferably be provided withan identifier for each item of information so that, if an item isupdated, it will be able to associate the item with its update.

In accordance with one aspect of the invention, in one embodiment, themass storage subsystems 14(n) provide access requests, including storagerequests and retrieval requests, to the remote data protection facility5 when information is to be stored in, or retrieved from, the remotedata protection facility 5, without requiring any action by a hostcomputer 11(n). In that embodiment, the generation of storage andretrieval requests is, accordingly, effectively transparent to the hostcomputers 11(n) and programs being processed thereby. The structure andoperation of one embodiment of a digital data processing system 11(n)useful in connection with the remote data protection facility 5 will bedescribed in connection with FIGS. 2 and 3, and the structure andoperation of the remote data protection facility 5 itself will bedescribed in connection with FIGS. 4 through 12.

II. Digital Data Processing System 10(n)

FIG. 2 depicts a functional block diagram of a digital data processingsystem 10(n) which is useful with the remote data protection facility 5.The digital data processing system 10(n) is generally similar to thedigital data processing system 10(n) described in the above-identifiedYanai patent and Vishlitzky Cache management application and Vishlitzkypatent applications. FIG. 3 depicts several data structures which areuseful in understanding the operation of the digital data processingsystem 10(n) depicted in FIG. 2. With reference to FIG. 2, digital dataprocessing system 10(n) includes a plurality of host computers 11(n)(1)through 11(n)(K) (generally identified by reference numeral 11(n)(k)),mass storage subsystem 14(n) and a remote data protection facilityinterface 17 interconnected by a common bus 13. Each host computer11(n)(k) includes a local computer 16(k), which may comprise, forexample, a personal computer, workstation, or the like which may be usedby a single operator, or a multi-user computer system which may be usedby a number of operators.

Each local computer 16(k) is connected to an associated host adapter15(k), which, in turn, is connected to bus 13. Each local computer 16(k)may control its associated host adapter 15(k) to perform a retrievaloperation, in which the host adapter 15(k) initiates retrieval ofinformation from the mass storage subsystem 14(n) for use by the localcomputer 16(k) in its processing operations. In addition, the localcomputer 16(k) may control its associated host adapter 15(k) to performa storage operation in which the host adapter 15(k) initiates storage ofprocessed data in the mass storage subsystem 14(n). Generally, storageoperations and retrieval operations in connection with the mass storagesubsystem 14(n) will collectively be referred to as "access operations."

The remote data protection facility interface 17 monitors storageoperations by the local computers's host adapters 15(k) and, when thehost adapter 15(k) initiates a storage operation as described below, itwill also receive the processed data and transfer it to the remote dataprotection facility 5 for mirrored storage. The remote data protectionfacility interface 17 can also initiate retrieval operations to retrieveinformation from the mass storage subsystem 14(n) to be transferred tothe remote data protection facility 5 for mirrored storage, as will alsobe described below. In addition, the remote data protection facilityinterface 17 can transfer operational commands to the remote dataprotection facility 5 to enable the remote data protection facility toperform predetermined operations. The operational commands may beprovided by, for example, a system manager through the host computers11(n)(1), or through a system manager console 19. Furthermore, theremote data protection facility interface 17 can also receiveinformation from the remote data protection facility 5 for storage inthe mass storage subsystem 14(n).

In connection with both retrieval and storage operations, the hostadapter 15(k) will transfer access operation command information,together with processed data to be stored during a storage operation,over the bus 13, and a bus access control logic circuit 18 is providedto arbitrate among devices connected to the bus, including the hostadapters 15(k), which require access to the bus 13. In controllingaccess to the bus 13, the bus access control logic circuit 18 may useany of a number of known bus access arbitration techniques, includingcentralized bus access control techniques in which bus access iscontrolled by one device connected to bus 13, as well as distributedarbitration techniques in which bus access control logic circuitry isdistributed among the devices which require access to the bus. Inaddition, the digital data processing system 10(n) includes the systemmanager console 19 which, in addition to permitting the system managerto control the remote data protection facility 10, also can permit asystem manager to control various elements of the system 10 in aconventional manner. It will be appreciated that, although the systemmanager console 19 is shown in FIG. 1 as a separate element, any of thelocal computers 22(h) may provide the functionality of the console 19,in which case a separate element need not be provided.

The mass storage subsystem 14(n) in one embodiment is generally similarto the mass storage subsystem described in U.S. Pat. No. 5,206,939,entitled System And Method For Disk Mapping And Data Retrieval, issuedApr. 27, 1993 to Moshe Yanai, et al (hereinafter, "the '939 patent"). Asshown in FIG. 1, the mass storage subsystem 14(n) includes a pluralityof digital data stores 20(1) through 20(M) (generally identified byreference numeral 20(m)), each of which is also connected to bus 13.Each of the data stores 20(m) stores information, including programs anddata, which may be accessed by the host computers 11 (n)(k) as well asprocessed data provided to the mass storage subsystem 14(n) by the hostcomputers 11(n)(k). Generally, the information is in the form ofrecords, which may be of variable length.

Each data store 20(m), in turn, includes a storage controller 21(m) andone or more storage devices generally identified by reference numeral22. The storage devices 22 may comprise any of the conventional magneticdisk and tape storage devices, as well as optical disk storage devicesand CD-ROM devices from which information may be retrieved. Each storagecontroller 21(m) connects to bus 13 and controls the storage ofinformation which it receives thereover in the storage devices connectedthereto. In addition, each storage controller 21(m) controls theretrieval of information from the storage devices 22 which are connectedthereto for transmission over bus 13. In addition to controlling accessby the host adapters 15(k) to bus 13, the bus access control logiccircuit 18 also controls access by the storage controllers to the bus13.

The mass storage subsystem 14(n) also includes a common memory subsystem30 for caching information during an access operation and event statusinformation providing selected status information concerning the statusof the host computers 11(n)(k) and the data stores 20(m) at certainpoints in their operations. The caching of event status information bythe common memory subsystem 34 is described in detail in U.S. patentapplication Ser. No. 08/532,240 filed Sep. 22, 1992, in the name of EliShagam, et al., and entitled Digital Computer System Including CommonEvent Log For Logging Event Information Generated By A Plurality ofDevices (Atty. Docket No. 95-034) assigned to the assignee of thepresent invention and incorporated herein by reference. The informationcached by the common memory subsystem 34 during an access operationincludes data provided by a host computer 11(n)(k) to be stored in adata store 20(m) during a storage operation, as well as data provided bya data store 20(m) to be retrieved by a host computer 11 (n)(k) during aretrieval operation. The common memory subsystem 34 effectively operatesas a buffer to buffer information transferred between the host computers11(n)(k) and the data stores 20(m) during a local access operation.

The common memory subsystem 30 includes a cache memory 31, a cache indexdirectory 32 and a cache manager memory 33, which are generallydescribed in U.S. patent application Ser. No. 07/893,509 filed Jun. 4,1992, in the name of Moshe Yanai, et al., entitled "System And MethodFor Dynamically Controlling Cache Management now U.S. Pat. No.5,381,539issued Jan. 19, 1995, " and U.S. patent application Ser. No.08/523,304, filed Sep. 5, 1995, in the name of Natan Vishlitzky, et al.,and entitled "Cache Management System Using Time Stamping ForReplacement Queue" (hereafter "Vishlitzky cache managementapplication"), now U.S. Pat. No. 5,592,432 issued Jan. 7, 1997, both ofwhich are assigned to the assignee of the present invention andincorporated herein by reference. The cache memory 31 operates as abuffer in connection with storage and retrieval operations, inparticular buffering records received from the host computers 11(n)(k)to be transferred to the storage devices for storage, and buffering datareceived from the data stores 20(m) to be transferred to the hostcomputers 11(n)(k) for processing.

The cache memory 31 and cache index directory 32 will generally bedescribed in connection with FIG. 3. With reference to FIG. 3, the cachememory 31 includes a series of storage locations, which are organized ina series of cache slots 35(0) through 35(S) (generally identified byreference numeral 35(s)). The storage locations are, in turn, identifiedby a series of addresses, with the starting address of a cache slotbeing identified by a base address. The cache slots 35(s), in turn,operate as the cache memory's buffer as described above.

The cache index directory 32 operates as an index for the cache slots35(s) in the cache memory 31. The cache index directory 32 includes aplurality of cache index tables 36(0) through 36(D) (generallyidentified by reference numeral 36(d)), each of which is associated withone of the storage devices 22 in the storage subsystem 14(n). Each cacheindex table 36(d) includes a device header field 40, which provides, forexample, selected identification and status information for the device22 associated with the cache index table 36(d). In addition, each cacheindex table 36(d) includes a plurality of cylinder descriptors 41(0)through 41(C) (generally identified by reference numeral 41(c)) each ofwhich is associated with one of the cylinders in the storage device 22that is associated with the cache index table 36(d). Each cylinderdescriptor 41(c), in turn, includes a cylinder header 42(c), whichprovides, for example, selected identification and status informationfor the cylinder associated with the cylinder descriptor 41(c).

In addition, each cylinder descriptor 41(c) includes a plurality oftrack descriptors 43(c)(0) through 43(c)(T) (generally identified byreference numeral 41 (t)), each of which is associated with one of thetracks in the cylinder 41(c). Each track descriptor 43(c)(t), in turn,includes information for the associated track of the storage device 22,including whether a copy of the data stored on the track is cached inthe cache memory 31, and, if so, the identification of the cache slot35(s) in which the data is cached. In particular, each track descriptor43(c)(t) includes a cached flag 44(c)(t) and a cache slot pointer45(s)(t). The cached flag 44(c)(t), if set, indicates that the data onthe track associated with the track is cached in a cache slot 35(s), andthe cache slot pointer 45(s)(t) identifies the particular cache slot inwhich the data is cached. In addition, each track descriptor 43(c)(t)includes a used flag 46(c)(t) which may be used to indicate whether thedata, after being stored in the cache slot identified by the cache slotpointer 43(c)(t), has been used by the host computer 11(n)(k) during aretrieval operation. This "host used" flag may be used to determinewhether the cache slot may be re-used for another access operation.

Each of the host adapters 15(k) and each of the storage controllers21(m) includes a cache manager 23(k) and 24(m), respectively, to accessto the cache memory 31, cache index directory 32 and cache managermemory 33. The particular operations performed during an accessoperation will depend on a number of factors, including the accessoperation to be performed, whether or not the data from the particulartrack to be accessed is cached in the cache memory 31, and whether ornot the data contained in a cache slot 35(s) has been modified orupdated by a host adapter's cache manager 24(k) during a storageoperation. As described in the aforementioned Vishlitzky cachemanagement application, the host computers 11(n)(k) typically performstorage and retrieval operations in connection with data in the cachememory 31, and the storage controllers 21(m) perform "staging" and"de-staging" operations to transfer data in the storage devices 22 tothe cache memory 31 for buffering (the staging operations) and totransfer data from the cache memory 31 to the storage devices 22 forstorage (the de-staging operations). In performing the staging andde-staging operations, the storage controllers 21(m) generally transferdata to and from the cache memory 31 in units of a track, that is, theywill during a staging operation transfer all of the data in a track froma storage device 22 to a cache slot 35(s) in the cache memory 31, andduring a de-staging operation copy all of the data in a slot in thecache memory 31 to the track of the storage device 22 from which it wasoriginally staged.

The cache manager memory 33 maintains a number of work lists which areused to control operations by the host adapters 15(k) and storagecontrollers 21(m) during an access operation. In particular, the cachemanager memory 33 includes a cache slot replacement list, a pendingwrite list and various lists which the host adapters 15(k) and storagecontrollers 21(m) use to communicate to coordinate staging operations(not shown). The various lists maintained by the cache manager memory 33may comprise any of a number of convenient forms, including queues,trees, stacks or the like. The cache slot replacement list is used tocontrol re-use of cache slots during staging operations in accordancewith a convenient cache-slot re-use methodology. During a stagingoperation, the storage controller's cache manager 24(m) uses the cacheslot replacement list to select a cache slot 35(s) into which it willload the data retrieved from a storage device. (The aforementionedVishlitzky cache management application describes a modifiedleast-recently-used cache-slot re-use methodology used in one embodimentof the invention). The pending write list is used to identify cacheslots 35(s) which contain updated data, which has not been written to astorage device. During de-staging operations, the storage controllers'cache managers 24(m) will use the write pending list to identify cacheslots to be written to a storage device 22. Preferably, the cache slots35(s) which are identified in the pending write list will not also belisted in the cache slot replacement list, so that cache slots 35(s)which contain updated data will not be used until the data has not beenwritten to a storage device through a de-staging operation.

The staging operation coordination communication lists include aplurality of stage request lists and a plurality of stage completionlists, with one stage request list being associated with each data store20(m) and one stage completion list being associated with each hostcomputer 11(n)(k). The host computers' cache managers 23(m) use thestage request lists to store stage requests to be performed by therespective data stores 20(m), and the data stores' cache managers 24(m)use the stage completion lists to store stage completion messages toindicate to the respective host computers' cache managers 23(m) that thestage requests have been completed.

Generally, a host computer 11(n)(k), during a retrieval operation,attempts to retrieve the data from the cache memory 31. However, if thedata is not in the cache memory 31, it will enable the storagecontroller 21(m) which controls the storage device 22 that contains thedata to be retrieved to "stage" the track which contains the data to beretrieved, that is, to transfer all of the data in the track whichcontains the data to be retrieved into a slot in the cache memory 31.After the data to be retrieved is in a slot in the cache memory 31, thehost computer 11(n)(k) will retrieve the data from the slot. Similarly,during a storage operation, the host computer 11(n)(k) will determinewhether the particular track into which the data is to be written is ina slot in the cache memory 31 and if so will store the data in the slot.However, if the data is not in the cache memory 31, the host computer11(n) will enable the cache manager 24(m) and storage controller 21(m)which controls the storage device 22 that contains the track whose datais to be updated to perform a staging operation in connection with thetrack, thereby to transfer the data in the track into a slot in thecache memory 31. After the data from the track has been copied into thecache memory 31, the host computer 11(n)(k) will update the data in thetrack.

The storage controller 21(m) generally attempts to perform a stagingoperation in connection with an empty slot in the cache memory 31.However, if the storage controller 21(m) may find that all of the cacheslots in the cache memory 31 are filled, it will in any case select oneof the slots to be used with the staging operation. Before transferringthe data from the track to the selected cache slot, it will determinewhether the data in the slot has been updated by a storage operation,and if so copy the data to the storage device 22 in a de-stagingoperation, and thereafter perform a staging operation to copy the datafrom the storage device to the selected cache slot. It will beappreciated that the storage controller 21(m) need only perform ade-staging operation in connection with a cache slot if the data in thecache slot has been updated, since if the data in the cache slot notbeen updated before the slot is re-used (which may occur if the a hostcomputer 11(n)(k) has only performed retrieval operations therewith),the data in the cache slot corresponds to the data in the storage device22.

More specifically, as described in the aforementioned Vishlitzky cachemanagement application, during a retrieval operation, the cache manager23(k) of the initiating host adapter 15(k) will initially access thecache index table 36(d) in the cache index directory 32 associated withthe storage device 22 in which the data to be retrieved is stored, inparticular accessing the track descriptor 36(c)(t) of the cylinderdescriptor 36(c) to determine, from the condition of the cached flag42(c)(t), whether the data from the track is cached in a cache slot35(s) in the cache memory. If the cached flag 42(c)(t) indicates thatdata from the track is cached in a cache slot 35(s), the cache manager23(k) uses the cache slot pointer 43(t) to identify the particular cacheslot 35(s) in which the data is cached and retrieves the required datafrom the cache slot 35(s).

On the other hand, if the cache manager 23(k) determines from the cachedflag 36(c)(t) that the data from the track is not cached in a cache slot35(s), it will generate a stage request to enable the storage controller21(m) for the storage device 22 which maintains the data to beretrieved, load the stage request in the stage request queue for thedata store 20(m) and notify the storage controller 21(m) that a stagerequest had been loaded in the stage request queue. At some point afterreceiving the notification, the storage controller 21(m) will retrievethe stage request and perform a staging operation in response thereto.In performing the staging operation, the storage controller 21(m) willretrieve the data from the requested track, use the above-describedcache slot replacement list to select a cache slot 35(s), load the datainto cache slot 35(s) and update the track descriptor 36(c)(t) in thecache index table 36(d) associated with the storage device 22 toindicate that the data from the track is in the cache slot 35(s), inparticular setting the cached flag 42(c)(t) and loading a pointer to thecache slot in the cache slot pointer 43(c)(t).

After the storage controller 21(m) has completed the staging operation,it will load a staging completed message in the stage completion list inthe cache manager memory 33 associated with the host computer 11(n)(k)which issued the staging request, and notify the host computer's cachemanager 23(k) that a stage completed message has been loaded therein. Atsome point after receiving the notification, the host computer's cachemanager 23(k) can repeat the operations performed in connection with theretrieval request as described above, in particular accessing the cacheindex table 36(d) in the cache index directory 32 associated with thestorage device 22 in which the data to be retrieved is stored, inparticular accessing the track descriptor 36(c)(t) of the cylinderdescriptor 36(c) to determine, from the condition of the cached flag42(c)(t), whether the data from the track is cached in a cache slot35(s) in the cache memory and, if so, use the cache slot pointer 43(t)to identify the particular cache slot 35(s) in which the data is cachedand retrieve the required data from the cache slot 35(s). Since at thispoint the cached flag 42(c)(t) should indicate that the data from thetrack is cached in a cache slot 35(s), the cache manager 23(k) should beable to complete the retrieval operation.

Similar operations occur during a storage operation, in which data in aparticular track is updated, with the additional operation of removingthe identification of the cache slot 35(s) containing data to be updatedfrom the replacement list and loading it into the pending write list.During a storage operation, the cache manager 23(k) of the initiatinghost adapter 15(k) will initially access the cache index table 36(d) inthe cache index directory 32 associated with the storage device 22 inwhich the data to be updated is stored, in particular accessing thetrack descriptor 36(c)(t) of the cylinder descriptor 36(c) to determine,from the condition of the cached flag 42(c)(t), whether the data fromthe track is cached in a cache slot 35(s) in the cache memory. If thecached flag 42(c)(t) indicates that data from the track is cached in acache slot 35(s), the cache manager 23(k) uses the cache slot pointer43(t) to identify the particular cache slot 35(s) in which the data iscached and loads the update data into the cache slot 35(s). In addition,the host adapter's cache manager 23(k) will remove the identification ofthe selected cache slot 35(s) from the replacement list to the pendingwrite list so that the cache slot 35(s) will not be re-used until ade-staging operation has been performed in connection with the cacheslot 35(s).

On the other hand, if the cache manager 23(k) determines from the cachedflag 36(c)(t) that the data from the track is not cached in a cache slot35(s), it will generate a stage request to enable the storage controller21(m) for the storage device 22 which maintains the data to beretrieved, load the stage request in the stage request queue for thedata store 20(m) and notify the storage controller 21(m) that a stagerequest had been loaded in the stage request queue. At some point afterreceiving the notification, the storage controller 21(m) will retrievethe stage request and perform a staging operation in response thereto.In performing the staging operation, the storage controller 21(m) willretrieve the data from the requested track, select a cache slot 35(s),load the data into cache slot 35(s) and update the track descriptor36(c)(t) in the cache index table 36(d) associated with the storagedevice 22 to indicate that the data from the track is in the cache slot35(s), in particular setting the cached flag 42(c)(t) and loading apointer to the cache slot in the cache slot pointer 43(c)(t).

After the storage controller 21(m) has completed the staging operation,it will load a staging completed message in the stage completion queuein the cache manager memory 33 associated with the host computer11(n)(k) which issued the staging request, and notify the cache manager23(k) that a stage completed message has been loaded therein. At somepoint after receiving the notification, the cache manager 23(k) canrepeat the operations performed in connection with the retrieval requestas described above, in particular accessing the cache index table 36(d)in the cache index directory 32 associated with the storage device 22 inwhich the data to be retrieved is stored, in particular accessing thetrack descriptor 36(c)(t) of the cylinder descriptor 36(c) to determine,from the condition of the cached flag 42(c)(t), whether the data fromthe track is cached in a cache slot 35(s) in the cache memory and, ifso, use the cache slot pointer 43(t) to identify the particular cacheslot 35(s) in which the data is cached and retrieve the required datafrom the cache slog 35(s). Since at this point the cached flag 42(c)(t)should indicate that the data from the tack is cached in a cache slot35(s), the cache manager 23(k) should be able to complete the storageoperation as described above.

As described above, the data stores'cache managers 24(m) also performde-staging operations using the pending write list to identify cacheslots 35(s) which contain updated data to be written back to theoriginal storage device 22 and track whose data was cached in therespective cache slots 35(s). After the data store's cache manager 24(m)has de-staged a cache slot 35(s), it will notify the remote dataprotection facility 20, which, in turn, will retrieve the records in thede-staged cache slot 35(s) and transfer them to the remote dataprotection facility 5 for storage. After it receives an acknowledgmentfor the records from the remote data protection facility 5, the remotedata protection facility 5 can remove the cache slot's identificationfrom the pending write list and return it to the replacement list sothat the cache slot 35(s) can be reused.

As indicated above, the remote data protection facility interface 17performs several operations. Generally, the remote data protectionfacility interface 17:

(i) monitors storage operations by the local computers's host adapters15(k) and, when a host adapter 15(k) initiates a storage operation, itwill also receive the processed data and transfer it to the remote dataprotection facility 5;

(ii) initiates retrieval operations to retrieve information from themass storage subsystem 14(n) for transfer to the remote data protectionfacility 5 for mirrored storage, and

(iii) receives information from the remote data protection facility 5for storage in the mass storage subsystem 14(n) during a reconstructionoperation.

In addition, the remote data protection facility interface 17 cantransfer operational commands provided by a system manager to the remotedata protection facility 5 to control the operations thereof. The remotedata protection facility interface 17 can also receive statusinformation representing the operational status of the remote dataprotection facility 5, which status information can be provided to asystem manager.

As will be described below in connection with FIGS. 4 through 13, theremote data protection facility 5 stores data from the digital dataprocessing systems 10(n) in the form of fixed-length portions which willbe referred to as "segments." In one embodiment, in which the storagedevices comprise disk storage units, each segment is selected tocomprise contents of an entire track of a respective storage device 22,which can include one or more CKD records as described above. Generally,when a host adapter 15(k) initiates a storage operation, the informationthat is stored will not comprise a complete segment of data (that is,data for a complete track) in the cache memory 31, but instead willstore only information comprising a partial segment. As described above,the remote data protection facility interface 17 will also transfer thispartial segment to the remote data protection facility 5 for mirroredstorage.

As will be described below, the remote data protection facility 5operates in two phases, including (i) filtering and bufferinginformation received from the digital data processing systems 10(n) andthereafter (ii) storing the filtered information on, in one embodiment,magnetic tape storage cartridges. During the filtering and bufferingphase, the remote data protection facility 5 will buffer all of theinformation that it receives from the remote data protection facilityinterface. However, at some point prior to storing the filteredinformation on the tape cartridges, the remote data protection facility5 will determine whether the particular information received from theremote data protection facility interface 17 comprises a partial segmentor a full segment, and if the information comprises a partial segmentthe remote data protection facility 5 will request the remote dataprotection facility interface 17 to provide the complete segment. Atthat point, the remote data protection facility interface 17 caninitiate a retrieval operation in connection with the mass storagesubsystem 14(n) to retrieve the segment. The operations performed by theremote data protection facility interface 17 and the mass storagesubsystem 14(n) during this retrieval operation are similar to thosedescribed above in connection with retrieval operations initiated by ahost computer's host adapter 15(n), and may necessitate performance of astaging operation as described above to enable the segment to be loadedin the cache memory 31. After the segment has been loaded in the cachememory 31, the remote data protection facility interface 17 can retrieveit and transfer it to the remote data protection facility 5.

When the full segment is received by the remote data protection facility5, the filtering performed during the filtering phase will preferablyprovide that the previously-received partial segment will befiltered-out and discarded. It will also be apparent from thedescription of the remote data protection facility 5 below that if it(that is, the remote data protection facility 5) has received any otherpartial segments for the particular segment prior to receiving the fullsegment from the remote data protection facility interface 17, thoseother partial segments will also be discarded, since the informationcontained therein will also be contained in the full segment receivedfrom the remote data protection facility interface 17. On the otherhand, if the remote data protection facility interface 17 is unable toprovide the full segment, which may occur, for example, as a result of amalfunction or other failure in connection with the mass storagesubsystem 14(n), the partial segment(s) may be useful in reconstructingthe full segment during a reconstruction operation.

III. Remote Data Protection Facility 5

A. General

The structure and operation of the remote data protection facility 5will be described in connection with FIGS. 1 and 4 through 13. Withreference initially to FIG. 1, the remote data protection facility 5generally includes an input module 50, a filter/buffer module 51, a tapelog module 52, a reconstruction module 53 and an output module 54, allunder control of a control module 55. The input module 50, as will bedescribed below in detail in connection with FIG. 4, receivesinformation from the respective digital data processing systems 10(n)which is to be stored by the remote data protection facility 5, couplesit to the filter/buffer module 51, and generates acknowledgments fortransmission to the digital data processing system 10(n) from which theinformation was received. In addition, the input module 50 receivedcontrol commands from the respective digital data processing systems10(n), and couples them to the control module 55. The control commandsmay, for example, enable the remote data protection facility 5 to beginstoring information from the digital data processing system 10(n),retrieve previously-stored information for transmission to the same oranother digital data processing system 10(n), and the like.

The filter/buffer module 51 performs the filtering and buffering phaseas described above. The filter/buffer module 51 buffers informationreceived from the input module, formats it into predetermined formatsfor storage, and filters the buffered information, as will be describedbelow in detail in connection with FIGS. 5 and 8. In one particularembodiment, the filter/buffer module 51 buffers the received informationusing one or more disk storage devices, although it will be appreciatedthat other digital data storage devices, such as conventional randomaccess memories, may be used in instead of the disk storage devices orto augment the storage provided by the disk storage devices. Ifinformation received from a digital data processing system 10(n) is inthe form of a partial segment, the filter/buffer module 51 at some pointduring the filtering and buffering operation will also request thesource digital data processing system 10(n), that is, the digital dataprocessing system 10(n) which provided the partial segment was received,to provide the entire segment.

After filtering by the filter/buffer module 51, the filtered informationis transferred to the tape log module 52 for storage. The tape logmodule 52 performs the storage phase as described above. In the tape logmodule 52, which will be described below in detail in connection withFIGS. 6, 10 and 11, the information received from the digital dataprocessing systems 10(n) is logged onto tape cartridges, such as digitallinear tape ("DLT") cartridges, using a conventional autochanger (notseparately shown) which forms part of the tape log module 52. In loggingthe information onto the tape cartridges, the tape log module 52 storesthe information received from the filter/buffer module 51 on acurrently-selected "logging" cartridge, without regard to whether theinformation currently being stored is an update of previously-storedinformation which may be stored on the same or another cartridge. Aswill be described below in connection with FIGS. 6, 10 and 11, the tapelog module 52 uses various data structures to determine, if multipleupdates for the same segment are stored on one or more tape cartridge inthe tape log module 52, which update was most recently received from therespective digital data processing system 10(n), and, thus, is the validupdate.

In one embodiment, the autochanger includes eighty-eight tape cartridgesdivided into eight groups, or "logging sets," of eleven cartridges each.The autochanger includes one robot arm, which is used to move cartridgesbetween cartridge storage slots in which the cartridges are normallystored and ones of nine drives for storing information on and retrievinginformation from the tape cartridges. Generally, one drive will beallocated for use with an associated one of the logging sets, and theninth drive will be used if a space reclamation operation is beingperformed in connection with a cartridge from one of the logging sets asdescribed below.

Each "protected volume" whose data is mirrored by the remote dataprotection facility 5 is associated with one logging set, although onelogging set may be associated with a number of protected volumes. In oneembodiment, each protected volume is associated with one of the storagedevices 22 in a mass storage subsystem 14(n). Each segment which isreceived by the remote data protection facility 5 is associated with asegment identifier that uniquely identifies the particular mass storagesubsystem 14(n), protected volume, cylinder and track on which thesegment is stored.

Periodically, the tape log module 52 will perform a space reclamationoperation in connection with a cartridge, as a space reclamation sourcecartridge, to consolidate all of the valid segment updates from thespace reclamation source cartridge onto one or more other cartridges.Preferably, a large portion of the segment updates on the spacereclamation source cartridge will be invalid, that is, they will havebeen superseded by more recently-received segment updates which may bestored on other cartridges. After the space reclamation operation, thespace reclamation source cartridge may be considered empty and used forstoring data during subsequent storage and space reclamation operations.During a space reclamation operation, the valid segment updates will becopied from the source cartridge onto the cartridge from the log setwhich is currently being used for logging, that is onto whichinformation from the filter/buffer module 51 is being stored. The spacereclamation operation will be performed concurrently with the loggingoperation, so that valid segment updates retrieved from the spacereclamation source cartridge will be stored on the current loggingcartridge interleaved with segment updates that are provided to the tapelog module 52 by the filter/buffer module 51. During a space reclamationoperation, if the current logging cartridge becomes filled anothercartridge may be selected as the current logging cartridge; accordingly,during a space reclamation operation, valid segment updates from thespace reclamation source cartridge may be copied onto several cartridgesin the logging set.

When a protected volume needs to be provided with information stored ina log set of the tape log module 52, in either a full reconstructionoperation or a partial reconstruction operation, the reconstructionmodule 53 will retrieve the required information and provide it to theoutput module 54, which, in turn, provides the information to theprotected volume's digital data processing system 10(n) or to anotherdigital data processing system 10(n'). In that operation, thereconstruction module 53 may obtain the information from the particularones of the cartridges on which the information has been stored by thetape log module 52, as well as from the filter/buffer module 51 if thatmodule 51 is buffering more recently received information than is storedon the cartridges. The reconstruction module 53 may perform a fullreconstruction operation if all of the information from, for example, aparticular digital data processing system 10(n) needs to bereconstructed, which may occur, for example, in the event of acatastrophic failure at the digital data processing system 10(n). On theother hand, the reconstruction module 53 may perform a partialreconstruction operation if information from only one or several storagedevices 22 (FIG. 2) needs to be provided, which can occur, for example,in the event of a failure by the storage devices 22.

During a reconstruction operation in connection with a protected volume,the reconstruction module 53 will enable the tape log module 52 to scanthrough the cartridges of the log set on which the information from theprotected volume is mirrored to retrieve the valid information for theprotected volume or volumes whose information is to be reconstructed. Inaddition, the reconstruction module 53 can retrieve information that iscurrently being buffered for the protected volume or volumes whoseinformation is to be reconstructed from the filter/buffer module 51 andmerge that information with the information retrieved from thecartridges. The reconstruction module 53 will provide the mergedinformation to the output module 54, which in turn will transmits theinformation to the protected volume's digital data processing system10(n), or to another digital data processing system 10(n') (n'≠n) if,for example, there was a catastrophic failure at the original digitaldata processing system 10(n).

The reconstruction module 53 can perform essentially two types ofreconstruction operations, namely, a full reconstruction operation and apartial reconstruction operation. In the embodiment in which a protectedvolume comprises a single storage device 22, in a partialreconstruction, the reconstruction module can perform a reconstructionoperation in connection with the mirrored information for the protectedvolume and provide the reconstructed information to the output module 54to be transferred to the mass storage subsystem 12(n) which contains theprotected volume. The mass storage subsystem 12(n), in turn, candistribute the information among its other storage devices, load it ontoa spare, or the like. During a partial reconstruction operation, thetape log module 52 can retrieve information from a plurality of thecartridges of the log set in parallel, in multiple ones of the drivesprovided by the autochanger, to reduce the time required for the partialreconstruction. A full reconstruction operation, in which all or asubset of protected volumes of one or more mass storage subsystems 14(n)will be reconstructed, is generally similar, except that information maybe retrieved from cartridges from multiple log sets. In any case, bylimiting storage of information from a single protected volume to asingle log set, the number of cartridges that need to be scanned toreconstruct the information from the protected volume can be limited,which, in turn, can also serve to reduce the time required for thepartial reconstruction.

As indicated above, the various elements 50 through 55 of the remotedata protection facility 5 operate under control of the control module55. The control module 55 controls the remote data protection facilityin response to commands received from the various digital dataprocessing systems 10(n), which may enable it to, for example, initiatelogging for a respective digital data processing system 10(n), andinitiate a full or partial data reconstruction operation, as will bedescribed below.

B. Input Module 50

FIG. 4 depicts the structure of the input module 50 useful in the remotedata protection facility 5. With reference to FIG. 4, the input module50 includes a plurality of interfaces 60(1) through 60(N) (generallyidentified by reference numeral 60(n)) each of which is connected toreceive information from a correspondingly-indexed digital dataprocessing system 10(n) over a communication link 12(n). Each interface60(n) receives signals, either in electrical or optical formrepresenting digital information or control commands that aretransmitted to the remote data protection facility 5, converts thesignals to digital form and provides the digitized information to arespective block generator 61(1) through 61(N) (generally identified byreference numeral 61(n)).

Each block generator 61(n), in turn, receives the digital informationprovided by the interface 60(n) and generates therefrom individual itemsof information, and in addition aggregates the individual items intoblocks to be logged. Each item of information, which will be termedherein a "segment update," corresponds to information from either apartial segment or a full segment, with, as indicated above, a fallsegment corresponding in one embodiment to the information stored on atrack of a storage device 22. Since a segment update may comprise apartial segment or a full segment, a segment update may be of variablelength, up to a maximum length which corresponds to the maximum amountof information that can be stored on a track of a storage device 22.Each segment update is associated with a segment identifier, which inone embodiment is a selected function of an identifier identifying themass storage subsystem 14(n) which contains the protected volume onwhich the segment associated with the update is stored, an identifierfor the protected volume itself, and an identifier for the track onwhich the segment update is stored. The segment identifier will remainconstant if the contents of the record are changed, modified or updated,for reasons which will be clear from the following description.

Each block generator 61(n) aggregates the received segment updates,along with the segment identifiers and other information, intofixed-sized blocks, which have structures which will be described belowin more detail in connection with FIG. 5. The blocks may have anyconvenient length; in one embodiment, in which the filter/buffer module51 buffers the information received from the digital data processingsystems 10(n) in disk storage devices, the block length is selected tobe greater than the maximum segment length. Each block accommodates atleast one segment update. In one embodiment, each segment update isstored in at most one block, so that segment updates will not be dividedacross multiple blocks. Each block generator 61(n), after generating thesegment updates and aggregating them into respective blocks, passes theblocks to the filter/buffer module 51 for buffering and filtering.

In addition, each block generator 61(n) receives the digital datarelating to control commands and generates command information therefromwhich it provides to the control module 55. The control information mayenable the control module to, for example, enable the filter/buffermodule 51, tape log module 52 and reconstruction module 53 to beginoperations in connection with a new digital data processing system, andto perform a full or partial reconstruction operation to reconstructinformation which it has been logging for a digital data processingsystem.

C. Filter/Buffer Module 51

FIG. 5 depicts the structure of the filter/buffer module 51 useful inthe remote data protection facility 5 (FIG. 1). With reference to FIG.5, the filter/buffer module 51 maintains a block queue 70, and a segmentidentifier hash table 71 all under control of a control module 73. Theblock queue 70 generally includes the blocks generated by the blockgenerators 61(n). The block queue 70 can have enqueued therewith avariable number of blocks. After each block has passed through thequeue, the filter/buffer module 51 selectively provides ones of thesegment update(s) in the block to the tape log module 52 for recordingon the respective tape log sets maintained thereby. In particular, whena segment update reaches the head of the block queue 70, if no morerecent segment update has been received by the filter/buffer module 51and enqueued with the block queue 70, the filter/buffer module 51 willprovide the segment update to the tape log module 52 for recording. Aswill be described below in connection with FIGS. 6, 10 and 11, for eachsegment update received by the tape log module 52, the tape log module52 in turn will store the segment update on the tape log set which isused to store segment updates for the particular protected volume withwhich the segment update is associated.

On the other hand, for each segment update for a segment, for which amore recent segment update has been received by the filter/buffer module51 and enqueued with the block queue 70, the filter/buffer module 51will discard the earlier-received segment update, and not provide it tothe tape log module for recording. As will be described below in greaterdetail, the filter/buffer module 51 will provide segment updates whichreach the head of the block queue 70 to the tape log module 52 forrecording, and so, if the later-received segment update reaches the headof the block queue 70 before the filter/buffer module 51 receives a yetlater segment update for the segment, the filter/buffer module 51 willprovide that later-received segment update to the tape log module 52 forrecording. On the other hand, if a yet later segment update is received,when the "later-received segment update" reaches the head of the blockqueue 70, that "later received segment update" will also be discarded.

The segment identifier hash table 71 is used to identify the particularblock of the block queue 70 which contains the most recently receivedsegment update for each segment for which a segment update is containedin a block of the block queue 70. Thus, when the filter/buffer module 51is to determine whether a segment update contained in a block of theblock queue 70 is the most recently received update, prior to providingthe segment update to the tape log module 52 for storage, it (that is,the filter/buffer module 51) will determine whether the segmentidentifier hash table identifies the block as containing the mostrecently-received segment update. On the other hand, if the segmentidentifier hash table 71 indicates, for a segment update in a block,that an updated copy of the segment update is stored in another block inthe filter/buffer module 51, that particular segment update in the blockwill not be passed to the tape log module 52 for storage. This willoccur for each segment update in each of the blocks in the block queue70, and so the filter/buffer module 51 will ensure that each blockpreferably remains in the queue for a period of time that issufficiently long that it is likely that, if the digital data processingsystem 10(n) updates the information contained in the segment againwithin some time after an update is stored in the queue, it will besuperseded or filtered out before the update is stored by the tape logmodule 51. In one embodiment, the time that a block remains in the blockqueue 70 is controlled to some extent by providing that the block queue70 will have at least a minimum number of blocks prior to providingsegment updates to the tape log module 52. This filtering will serve toreduce the number of segment updates associated with a particularsegment (that is, which are associated with a particular segmentidentifier) which are stored by the tape log module 52 if the digitaldata processing system 10(n) modifies the record several times within arelatively short period of time.

More specifically, the block queue 70 contains the various blocks thatare generated by the block generators 61(n) and provided to thefilter/buffer module 51. Block queue 70 comprises a block queue header74 and a series of one or more block queue elements 75(1) through 75(B)(generally identified by reference numeral 75(b)), with block queueelement 75(1) comprising the "head" of the block queue 70 and blockqueue element 75(B) comprising the "tail" of the block queue 70. Theblock queue header 74 includes two fields, including a head pointerfield 80 and a tail pointer field 81, with the head pointer field 80pointing to the head block queue element 75(1), and the tail pointerfield 81 pointing to the tail block queue element 75(B). Each successiveblock queue element 75(1) through 75(B-1) points to the next block queueelement in the series of block queue elements comprising the block queue70, thereby to define and establish the series of block queue elementsdefining the block queue 70. The tail block queue element 75(B) maycontain a null or other value which indicates that it is the last blockqueue element in the block queue 70.

Each block queue element 75(b), in turn, comprises a queue elementheader 76 and a block 77. The queue element headers 76 of the respectiveblock queue elements 75(b) essentially serve to define the order of theblock queue elements 75(b) in the block queue 70 and identify therespective blocks that are associated with the queue 70. The block 77associated with each queue element header 76 generally corresponds toone of the blocks that is generated by the record generator 61(n) of theinterface 50 (FIG. 4). Each queue element header 76 includes severalfields, including at least a next block pointer field 82 and a blockpointer field 84. The next block pointer field 82 in header 76 of ablock queue element 75(b) contains a next block pointer to the nextblock queue element 75(b+1) in the block queue 70, and thus the nextblock pointers effectively serve to define the order of the block queue70 as described above.

The block pointer field 84 includes a block pointer that points to theblock 77 that is associated with the block queue element 75(b). In theembodiment in which the filter/buffer module 51 buffers the informationreceived for the protected volumes in a disk storage device, the blockpointer will preferably comprise the address of the storage location inthe disk drive unit in which the block 77 is stored; it will beappreciated, however, that if other storage media, such as conventionalrandom access memories, are used to store the information, the blockpointer in field 84 will generally contain an address that identifiesthe location in the storage media in which the block 77 is stored.

Block 77, which, as indicated above, is pointed to by the block pointer84, includes one or more entries 85(1) through 85(R) (generallyidentified by reference numeral 85(r)), with each entry 85(r) beingassociated with one segment update loaded into the block by the blockgenerator 61(n). Each entry 85(r), in turn, includes a number of fields,including a segment identifier field 90, a segment length field 91, anda segment update information storage field 93. The actual segment updateis stored in the segment update information storage field 93. Thesegment identifier field 90 receives the segment identifier for thesegment update. As indicated above, the segment update can be ofvariable length, and the segment length field 91 stores a segment lengthvalue that identifies the length of the segment update. As describedabove, the segment update can be either a full segment or a partialsegment, and it will be appreciated that, in addition to helpingidentify the beginning of the next entry 85(r) in the block 77, thesegment length value in the segment length field 91 can also be used doindicate whether the segment update stored in the entry 85(r) comprisesa partial segment or a full segment.

As indicated above, the filter/buffer module 51 also provides a sourceof queue elements (not shown). The queue element source may be in theform of, for example, queue which buffers unused queue elements untilthey are required for use in the block queue 70. Thus, when a new blockqueue element is required for a new block received by the filter/buffer51 from a record generator 61(n) of input 50, the queue element will beprovided by the queue element source for use in establishing a blockqueue element 75(b). In addition, when segment updates from a block 77associated with a block queue element 75 have been either transferred tothe tape log module 52 for storage or discarded, the queue element 75(b)is returned to the queue element source.

As noted above, the filter/buffer module 51 also includes the segmentidentifier hash table 71 which identifies the block queue elements75(b), and thus the blocks 77, which contain the most recently receivedsegment updates. The record identifier hash table 71 includes a selectednumber of pointer entries 110(1) through 110(H) (generally identified byreference numeral 110(h)) which point to respective linked lists 111(h),with each index "h" representing a hash value which may be generated byapplying a selected hash function to the segment identifiers for thevarious segment updates that may be received by the remote dataprotection facility 5 from the digital data processing systems 10(n).Each pointer entry 110(h) contains a pointer that points to thecorrespondingly-indexed linked list 111(h).

Each linked list 111(h), in turn, can comprise one or more list entries111(h)(1) through 111(h)(J) (generally identified by reference numeral111(h)(j)) which are associated with a various segment updates for thevarious segments whose segment identifiers hash to the hash valuecorresponding to the index "h." Each entry in list 111(h) comprises anumber of fields, including a segment identifier field 112, a blockqueue entry pointer field 113 and a next hash entry pointer field 114.When a new block 77 is received from the block generator 61(n) and usedin a new block queue element 75(b), for each segment update in theblock, a hash function is applied to the segment update's segmentidentifier to provide a hash value "h," which is used as an index toidentify a pointer entry 110(h) in the segment identifier hash table 71.If the pointer entry 110(h) contains a null or other value thatindicates that there is no list 111(h) associated with the pointer entry110(h), which can occur if the block queue 70 does not contain anyblocks 77 which, in turn, contain segment updates whose segmentidentifier hashes to the entry's index value "h," a list 111(h) will beestablished by creating a new list entry 111(h)(1) for the segmentupdate. In addition, the segment identifier for the segment update willbe loaded into the segment identifier field 112, and a block pointerloaded into the block queue entry pointer field 113 to point to theblock 77 of the block queue 70 that contains the segment update. Inaddition, a null or other value may be provided in the next hash entrypointer field 114 to indicate that the entry is the last entry 111(h)(J)in the list 111(h).

On the other hand, if the pointer entry 110(h) contains a pointer to alist 111(h), the list 111(h) contains one or more entries 111(h)(j)whose segment identifier values hash to the index value "h." One of theentries 111(h)(j) in that list may contain a segment identifier field112 which contains a segment identifier that corresponds to the segmentupdate's segment identifier. In that case, the entries 111(h)(j) in thelist 111(h) can be scanned to determine whether it contains an entry forwhich the segment identifier field 112 contains a segment identifiervalue that corresponds to the segment identifier for the new segmentupdate. If so, the block pointer field 113 for that entry 111(h)(j) canbe updated to point to the new block, which, in turn, will ensure thatthe segment identifier hash table 71 will always point to the block 77which contains the most recently received segment update for aparticular segment identifier. On the other hand, if the list 111(h)does not contain an entry 111(h)(j) for which the segment identifierfield 112 contains a segment identifier value that corresponds to thesegment identifier for the new segment update, a new entry 111(h)(j) canbe added to the list 111(h) in a manner similar to that described above,and linked to the list 111(h) by loading a pointer pointing to the newentry 111(h)(j) in the next pointer field 114 of the last entry in thelist 111(h).

As described above, the filter/buffer 51 also includes a control module73 to control the block queue 70 and the segment identifier hash table71 to receive blocks from the input module 50, establish block queueelements 75(b) therefor, and selectively transfer segment updates fromthe block queue 70 to the tape log module 52 for storage. In addition,the control module 73 will determine whether segment updates stored inthe block queue elements 75(1) are partial segments and, if so, willenable a request to be transmitted to the remote data protectionfacility interface 17 of the appropriate digital data processing system10(n) (FIG. 1) to initiate the retrieval of the corresponding fullsegments. Preferably, for each such partial segment in the block queue70, the control module 73 will issue a request for the correspondingfull segment so that it would normally receive the full segment beforethe block queue element containing the partial segment reaches the headof the block queue. It will be appreciated description that, when thefull segment is received, the entry 111(h)(j) in the segment identifierhash table will be updated to point to the block queue entry 75(b) whichcontains the full segment, in which case the partial segment will bediscarded and not passed to the tape log module 52 for storage. In oneembodiment, if the full segment is not so received, the partial segmentwill not be passed to the tape log module 52 (in that embodiment, onlyfull segments are stored by the tape log module 52), but instead theremote data protection facility 5 marks the segment as being invalid andwill not be reconstructed by the reconstruction module 53 during areconstruction operation.

Generally, the control module 73, when a block 77 is received from ablock generator 61(n), forms a block queue element 75(b) and enqueues it(that is, the block queue element) to the block queue. In thoseoperations, in response to receipt of a block from a block generator61(n), the control module 73 will:

(a) retrieve a queue element from the queue element source, generate ablock queue element and link the generated block queue element as thetail block queue element 75(B) for the block queue 70, and

(b) update the segment identifier hash table 71 to enable the respectiveentries of the lists 111(h) whose segment identifiers identify thesegment updates in the new tail block queue element 75(B) to point tothe new tail block queue element 75(B).

In generating a block queue element and linking it as the tail blockqueue element 75(B) (item (a) above), the control module 73 will updateboth (i) the tail pointer 81 of the block queue's block queue header 74and (ii) the next block pointer 82 of the block queue element which waspreviously at the tail of the block queue 70, to point to the new tailblock queue element 75(B). The control module 73 will also condition theblock queue header 76 of the new tail block queue element 75(B), inparticular, (i) providing an appropriate value as the next block pointer82, which, as noted above, may illustratively comprise a null value);and (ii) providing a pointer for the block pointer field 84 which pointsto the new block 77. The control module 73 will perform these operationsfor each of the blocks received from the block generator 61(n) of theinput module 50 (FIG. 4)

As indicated above, the filter/buffer module 51 preferably maintains atleast a minimum number of block queue elements 75(b) in the block queue70 to ensure that the block queue elements 75(b), and thus the segmentupdates stored therein, will remain in the filter/buffer module 51 forat least some time before they are transferred to the tape log module 52for storage. Thus, while the block queue 70 contains at least therequired minimum number of block queue elements 75(b), the controlmodule 73 will selectively provide the segment updates from entries85(r) of the head block queue element 75(1) to the tape log module 52for storage on the appropriate tape log set. In that operation, thecontrol module 73, for each entry 85(r) of the block 77 contained in thehead block queue element 75(1), will determine whether the segmentupdate contained in the entry is the most recently received segmentupdate for the segment.

In making that determination, the control module 73 will, in turn, usethe selected hash function as described above to generate the hash value"h" for the segment identifier in field 90 of the entry 85(r) anddetermine whether the list 111(h) associated with that hash value "h" inthe segment identifier hash table 71 contains an entry whose blockpointer 113 points to the head block queue element's block 77. If thecontrol module 73 determines that the list 111(h) associated with thathash value "h" in the segment identifier hash table 71 contains an entrywhose block pointer 113 points to the head block queue element's block77, it can determine that the segment update contained in the entry85(r) is the most recently-received segment update for the segmentidentified in field 90, and provide that entry 85(r) to the tape logmodule 52 for storage. On the other hand, If the control module 73determines that the list 111(h) associated with that hash value "h" inthe segment identifier hash table 71 contains an entry whose blockpointer 113 points to the block 77 of a different queue entry 75(b)(b≠1), it can determine that the segment update contained in the entry85(r) is not the most recently-received segment update for the segmentidentified in field 90, and discard that entry 85(r).

With this background, the detailed operations enabled by the controlmodule 73 will be described in connection with the flow charts depictedin FIGS. 8 and 9, with FIG. 8 depicting operations performed by thecontrol module 73 when a block is received from the input module 50, andFIG. 9 depicting operations performed by the control module 73 inconnection with transferring of entries 85(r) from the head block queueelement 75(1) to the tape log module 52. With reference initially toFIG. 8, the control module 73 will

(i) receive a block 77 from the input module 50 (step 200);

(ii) retrieve a queue element from the queue element source (step 201);

(iii) enqueue the queue element, which was retrieved in step 201, to theblock queue 70 (step 202), in the process updating the next blockpointer field 82 of the current tail block queue element 75(B) and thetail pointer 81 of the header 74 of the block queue; and

(iv) load a pointer to the block 77 received from the input module 50into the block pointer field 84 (step 203) thereby to link the block 77to the tail block queue element 75(B).

Thereafter, the control module 73 will update the segment identifierhash table for each of the entries 85(r) in the block 77 of the newqueue element, and in those operations will:

(v) select the first entry 85(r) in the block 77 (step 204),

(vi) use the selected hashing function in connection with the segmentidentifier in field 90 of the selected entry to generate a hash value"h" (step 205); and

(vi) scan the list 111(h) of the segment identifier hash table 71pointed to by list pointer 10(h), to determine whether an entry existswhose segment identifier field 112 contains the same segment identifieras the segment identifier field 90 of the selected entry 85(r) (step206).

(vii) If the control module 73 makes a positive determination in step206, it will update the block pointer field 113 of the entry to point tothe block 77 of the new block queue element (step 207), but

(viii) if the control module makes a negative determination in step 206,it will generate a new entry for the list 111(h) and insert the segmentupdate's segment identifier in segment identifier field 112 and apointer to the block 77 of the new block queue element in block pointerfield 113 of the new entry (step 208)

(ix) Thereafter, the control module 73 will determine whether the block77 of the new block queue element contains any additional entries 85(r)(step 209), and

(x) in response to a positive determination in step 209, select the nextentry 85(r) (step 210) and return to step 205 to process that entry.

FIG. 9 depicts operations performed by the control module 73 inconnection with transferring of entries 85(r) from the head block queueelement 75(1) to the tape log module 52. In connection with thoseoperations, the control module 73 will:

(i) determine whether the block queue 70 has the required minimum numberof block queue elements 75(b) (step 220);

(ii) in response to a positive determination in step 220, obtain thepointer to the block 77 associated with the head block queue element75(1) from block pointer field 84 of the head block queue element'sblock queue header 76 (step 221);

(iii) select the first entry 85(r) in the block 77 pointed to by thepointer obtained in step 221 (step 222);

(iv) use the selected hashing function in connection with the segmentidentifier in field 90 of the selected entry to generate a hash value"h" (step 223);

(v) scan the list 111(h) of the segment identifier hash table 71 pointedto by list pointer 110(h), to identify the entry of list 111(h) whosesegment identifier field 112 contains the same segment identifier as thesegment identifier field 90 of the selected entry 85(r) (step 224); and

(vi) determine whether the block pointer in field 113 of the identifiedentry in list 111(h) points to the block 77 associated with the headblock queue element 75(1) (step 225).

(vii) In response to a positive determination in step 225, the controlmodule 73 will provide the contents of the selected entry 85(r),including the segment identifier in field 90, the segment identifier infield 91 and the segment update in field 93, to the tape log module 52for storage (step 226).

It will be appreciated that, if the control module 73 makes a positivedetermination in step 225, the entry in list 111(h) whose segmentidentifier in field 112 corresponds to the segment identifier in theentry 85(r) points to the block 77 associated with the head block queueentry 75(1). In that case, as described above, the head block queueentry 75(1) contains the most recently-received segment update for thesegment identifier, and so the control module 73 enables the segmentupdate and associated information in entry 85(r) to be transferred tothe tape log module 52 for storage.

Following step 226, or step 225 if the control module 73 makes anegative determination in that step, the control module 73 will

(viii) determine whether there are any additional entries 85(r) in theblock 77 (step 227), and

(ix) in response to a positive determination in step 226, select thenext entry 85(r) (step 228) and return to step 223 to initiateprocessing in connection with that entry 85(r).

It will be appreciated that, if the control module 73 makes a negativedetermination in step 225, the entry in list 111(h) whose segmentidentifier in field 112 corresponds to the segment identifier in theentry 85(r) does not point to the block 77 associated with the headblock queue entry 75(1). In that case, as described above, another blockqueue entry 75(b) will contain a more recent segment update for the samesegment identifier, and so the control module 73 selects the next entry85(r) in the block, if such an entry exists, and processes that entry.As a result, the control module 73 will effectively discard the contentsof the previously-selected entry 85(r).

(xii) Returning to steps 227, if the control module makes a positivedetermination in those steps, it will effectively dequeue the head blockqueue element 75(1) from the block queue 70 and provide it to the queueelement source (step 229).

As will be described below in connection with FIG. 6, the tape logmodule 52 includes a plurality of tape log sets, each of which isprovided with an input queue in which is enqueued segment updates whichwill be recorded onto tape cartridges in the respective tape log set. Inperforming step 226, the control module 73 can enqueue the selectedentry 85(r) to the input queue that is associated with the tape log setassociated with the protected volume from which the segment update wasreceived.

D. Tape Log Module 52

The tape log module 52 (FIG. 1) receives the segment updates passedthereto by the filter/buffer module 51 and stores them on theappropriate tape log sets associated with the respective protectedvolumes. The tape log module 52 will be described in connection withFIG. 6, which depicts elements of the tape log module 52. One tape logset, identified by reference numeral 130, and associated elements aredepicted in FIG. 6. As described above, in one embodiment the tape logmodule 52 comprises a plurality of tape log sets, each of which will beprovided with a number of the elements depicted in FIG. 6 as describedbelow.

Generally, the tape log module 52 performs two general types ofoperations, namely,

(i) a storage operation, in which segment update information providedthereto by the filter/buffer module 51 is stored on a tape cartridge ofthe tape log set 130; and

(ii) a "space reclamation" operation, in which selected "valid" segmentupdates are copied from one tape cartridge of a log set 130 to anothercartridge of the same log set, after which the tape cartridge from whichthe valid segment updates were copied is deemed empty.

Operations performed by the tape log module 51 in connection with eachof these will be described in connection with FIGS. 6, 10 and 11.

Generally, in performing a storage operation (item (i) above) the tapelog module 52 will store each segment update that it receives from thefilter/buffer module 51 on tape cartridges of the appropriate tape logset, that is, the tape log set to which the protected volume associatedwith the segment update is assigned. At any particular point in time,one tape cartridge of a tape log set 130 will be selected as a "currentlogging tape cartridge" onto which segment updates will be stored, andwhen the current logging tape cartridge is full, another tape cartridgefrom the same log set 130 will be selected as the current logging tapecartridge.

In performing a space reclamation operation (item (ii) above), the tapelog module 52 will copy "valid" segment updates, that is, the mostrecently received segment updates for their respective segments, fromone tape cartridge in the tape log set 130, as a "source" cartridgeduring the space reclamation operation, onto one or more of the othertape cartridges in the tape log set. Preferably, the valid segmentupdates from the space reclamation source cartridge will be copied ontothe current logging tape cartridge used for the storage operation (item(i) above). Indeed, the tape log module 52 will perform a spacereclamation operation (item (ii) above) concurrently with a storageoperation (item (i) above) and the valid segment updates from the spacereclamation source cartridge will be stored on the current logging tapecartridge interleaved with the segment updates from the filter/buffermodule 51. As indicated above, in one embodiment the tape log moduleprovides nine drives for storing information on and retrievinginformation from the tape cartridges, with one drive normally being usedwith each of the eight log sets. In that embodiment, the ninth drive isnormally used to retrieve segment updates from a space reclamationsource cartridge during a space reclamation operation.

After a tape cartridge has undergone a space reclamation operation as asource cartridge, it will be deemed empty and can be used as a currentlogging tape cartridge during a storage operation (item (i) above).Preferably, the tape log module 52 will select a tape cartridge in thelog set 130 to undergo a space reclamation operation as a sourcecartridge when the number of empty tape cartridges falls below apredetermined threshold number. In addition, preferably the tape logmodule 52 will select as the source cartridge the cartridge whichcontains the least number of valid segment updates. Operations performedby the tape log module 52 in selecting a cartridge to be a sourcecartridge in a space reclamation operation, and in identifying the onesof the segment updates on the selected source cartridge which are valid,will be described in detail below.

With this background, the structure of the tape log module 52 will bedescribed in connection with FIG. 6. With reference to FIG. 6, the tapelog module 52 includes, in addition to the tape log sets, an input queue132 associated with each tape log set for buffering segment updates tobe stored on the respective tape log set 130. In addition, the tape logmodule 52 provides several data structures which identify the segmentupdates stored on the various tape cartridges of the respective tape logsets 130 and which can be used in identifying valid segment updatesduring a space reclamation operation and a reconstruction operation. Thevarious elements of the tape log module 52 all operate under control ofa tape log control module 135. The tape log module 52 may make use of apool of queue elements (not shown), similar to the queue element sourceused with the filter/buffer module 51 as described above, which willprovide queue elements which will be used as described below.

Generally, the element identified as the tape log set 130 represents thetape cartridges used in the log set, the drives used to recordinformation on and retrieve information from the tape cartridges, andany electronic circuitry which is used in connection therewith. Eachtape log set 130 includes a number of tape cartridges 136(1) through136(M) (generally identified by reference numeral 136(m)) can be usedvariously for storing segment updates during a storage operation (item(i) above) and as a space reclamation source cartridge during a spacereclamation operation (item (ii) above).

The input queue 132 associated with a tape log set 130 providesbuffering for segment updates to be stored on the tape log set 130,including both segment updates provided by the filter/buffer module 51and those valid segment updates which are retrieved from a spacereclamation source cartridge during a space reclamation operation. Theinput queue 132 comprises a plurality of input queue elements 132(i),each of which stores the segment update, identifier and lengthinformation for one segment update received from the filter/buffermodule 51 for storage on the tape log set 130, or correspondinginformation retrieved from a space reclamation source cartridge during aspace reclamation operation. Each input queue element 132(i) alsoprovides an identifier (not separately shown) which identifies thesource of the information contained in the input queue element, that is,whether the information was received from the filter/buffer module 51 ora space reclamation source cartridge. In addition, the input queue 132provides queue header structures similar to those provided by header 74for the block queue 70 (FIG. 5), and each input queue element 132(i)also includes queue element header structures similar to those providedby header 76 for the respective block queue element 75(b) (FIG. 5).

As noted above, the tape log module 52 provides a number of datastructures which identify the segment updates stored on the various tapecartridges of the respective tape log sets 130 and which can be used inidentifying valid segment updates during a space reclamation operationand a reconstruction operation. The data structures include, for eachtape cartridge 136(m) in the tape log set, a cartridge segment directory141(m) and a segment bit map 142(m) which are used both during a spacereclamation operation and a reconstruction operation. In addition, thetape log module 52 includes an auxiliary segment bit map 143 which isused during a space reclamation operation.

As shown in FIG. 6, the cartridge segment directory 141(m) includes aseries of entries generally identified by reference numeral 160(e),which are generally associated with respective ones of the series ofsegment updates that are recorded on the associated tape cartridge ofthe tape log set 130. Thus, the "e-th" entry of the cartridge segmentdirectory 141(m) is associated with the "e-th" segment update that isrecorded on the tape cartridge 136(m) associated with the cartridgesegment directory 141(m). Each directory 141(m) identifies the series ofrecords that are stored on the tape cartridge, as well as their validand invalid status, that is, whether each is the most recently receivedsegment update for the segment and thus will survive space reclamationwhen the associated cartridge is selected as a space reclamation sourcecartridge. Each entry 160(e) includes three fields, including a validflag 161, an invalid flag 162 and a segment identifier field 163. Forthe respective entry 160(e), the valid and invalid flags 161 and 162 areconditioned indicate the valid or invalid status of the segment updateas recorded on the tape cartridge 136(m) associated with the cartridgerecord directory 141(m). The segment identifier field 163 contains asegment identifier value, and identifies the segment that is associatedwith the entry 160(e). As will be apparent below, the cartridge segmentdirectory 141(m) will not need to identify the specific location on thetape cartridge 136(m) at which each segment update is recorded on thetape cartridge.

As indicated above, the valid and invalid flags 161 and 162 of therespective entries 160(e) of the cartridge segment directory 141(m),along with the segment bit maps 142(m), are used to identify the validsegment updates for the various segments that are stored on the tape logset 130. In addition, the auxiliary segment bit map 143 is used during aspace reclamation operation to assist in identifying ones of the segmentupdates from the space reclamation source cartridge which are invalid.

In particular, each segment bit map 142(m) includes a plurality of bitseach associated with one of the possible segment identifiers for whichsegment updates may be stored on the tape cartridges in the log set 130.For each tape cartridge 136(m), the associated segment bit map 142(m)identifies the segment identifiers for the valid segment updates thatare stored on the tape cartridge 136(m). Thus, when a segment update fora particular segment is stored on a tape cartridge 136(m) during astorage operation, if the segment update is valid, the bit of theassociated segment bit map 142(m) associated with the segment will benormally be set to indicate that that cartridge 136(m) contains a validsegment update and corresponding bits in the segment bit maps 142(m')(m'≠m) associated with other tape cartridges 136(m') (m'≠m) will becleared to indicate that segment updates for the segment that may bestored on those cartridges 136(m') (m'≠m) are invalid.

It will be appreciated that, if the cartridge 136(m) contains multiplesegment updates for the segment, the segment bit map 142(m) does notidentify the particular segment update for the segment which is valid.As will be described in more detail below, if the tape cartridge 136(m)contains multiple segment updates for a particular segment identifier,the particular segment update that is valid is the segment update thatis associated with the last entry 160(e) of the cartridge segmentdirectory 141(m) for which the invalid flag 162 is not set. Thus, toidentify the segment updates from a space reclamation source cartridge136(s) which are to be selected for copying during a space reclamationoperation, the cartridge segment directory 141(s) associated with thesource cartridge 136(s) can be processed in reverse order, from the lastentry 160(E) to the first entry 160(1) to identify the last entry 160(e)in the directory 141(m) whose invalid flag 162 is not set for eachsegment whose bit is set in the segment bit map 142(m). The valid flagsfor the identified entries 160(e) will then be set to, in turn, identifythe particular entries which are associated with segment updates whichare candidates for retrieval from the space reclamation source cartridge136(s).

After the segment updates which are candidates for retrieval from thespace reclamation source cartridge 136(s) have been identified asdescribed above, the space reclamation source cartridge can be loadedinto a drive and segment updates, along with associated segmentidentifier and length information, retrieved therefrom. The validsegment updates can thereafter be selectively used to generate inputqueue elements 132(i) which can be enqueued to the input queue 132 forstorage on the current logging tape cartridge 136(1). In that operation,in addition to the cartridge segment directory 141(s) associated withthe space reclamation source cartridge 136(s), the auxiliary segment bitmap 143 will also be used in determining whether input queue elements132(i) are to be generated for the valid segment updates that areretrieved.

As with the segment bit maps 142(m), the auxiliary segment bit map 143comprises a series of bits each associated with one of the possiblesegment identifier values. At the beginning of a space reclamationoperation, the auxiliary segment bit map 143 is established andinitialized, and at the end of a space reclamation operation theauxiliary segment bit map 143 that was used during the operation may bediscarded. During a space reclamation operation, when a segment updatethat is received from the filter/buffer module 51 is stored on anycartridge 136(m) in the tape log set, the particular bit of theauxiliary segment bit map 143 associated with the segment update'ssegment identifier will be set. As described above, when each segmentupdate is stored on a tape cartridge currently being used for logging,the bit of the segment bit map 142(m) that is associated with the tapecartridge currently used for logging will also be set, therebyindicating that that tape cartridge contains the most recently receivedsegment update for the segment, but it will be appreciated that, asdescribed above, tape cartridges used for logging may be changed duringa space reclamation operation, and the auxiliary segment bit map 143will essentially provide a composite of identifiers for the segmentupdates that are received from the filter/buffer module 51 and stored onthe tape cartridge(s) that are used for logging during the spacereclamation operation.

The auxiliary segment bit map 143 is used twice during a spacereclamation operation, as follows. It will be appreciated that a segmentupdate received for a segment identifier from the filter/buffer module51 will be a more recent update than the segment update for the samesegment identifier that may be stored on the space reclamation sourcecartridge 136(s). Accordingly, when each segment update which haspreviously been identified as a candidate for retrieval (as describedabove) is actually retrieved from the space reclamation source cartridge136(s), prior to establishing an input queue element 132(i) therefor theparticular bit of the auxiliary segment bit map 143 associated with thesegment update's segment identifier is tested to determine whether it isset, indicating that a segment update for that identifier received fromthe filter/buffer module 51 has been stored on a tape cartridge duringthe space reclamation operation. This may occur if, for example, asegment update received from the filter/buffer module 51 was stored on atape cartridge 136(m) (m≠s) selected for logging after the segmentupdates identified as candidates for retrieval have been determined. If,for a candidate segment update retrieved by from the space reclamationsource cartridge 136(s), the associated bit of the auxiliary segment bitmap 143 is set, the candidate segment update can be discarded without aninput queue element 132(i) being established therefor.

On the other hand, if, for a candidate segment update retrieved by fromthe space reclamation source cartridge 136(s), the associated bit of theauxiliary segment bit map 143 is clear, indicating that no segmentupdate for that identifier received from the filter/buffer module 51 hasbeen stored on a tape cartridge during the space reclamation operation,an input queue element 132(i) will be established therefor.

The auxiliary segment bit map 143 is also used during a spacereclamation operation as follows. As indicated above, if, for acandidate segment update retrieved by from the space reclamation sourcecartridge 136(s), the associated bit of the auxiliary segment bit map143 is clear, indicating that no segment update for that identifierreceived from the filter/buffer module 51 has been stored on a tapecartridge during the space reclamation operation, an input queue element132(i) will be established therefor. However, it is possible that asegment update provided by the filter/buffer module 51 be present in theinput queue 132 awaiting storage on a tape cartridge 136(m) (m≠s) whenthe determination is made as to whether to establish an input queueelement 132(i) for a segment update retrieved from the space reclamationsource cartridge. In that case, the input queue element 132(f)containing the segment update received from the filter/buffer module 51will be ahead of the input queue element 132(s) containing the segmentupdate retrieved from the space reclamation source cartridge 136(s) inthe input queue 132 (that is, index "f" is less than index "s"), and thesegment update retrieved from the space reclamation source cartridge136(s) will be stored after the segment update received from thefilter/buffer module 51.

However, it will be appreciated that the earlier-stored segment update(that is, the segment update received from the filter/buffer module 51)will be more recent than the later-stored segment update retrieved fromthe space reclamation cartridge. Accordingly, to ensure that theearlier-stored segment update (that is, the segment update received fromthe filter/buffer module 51) will not be deemed invalid over thelater-stored segment update retrieved from the space reclamation sourcecartridge 136(s), the invalid bit 162 is set in the entry 160(e) of thecartridge segment directory 141(m) that is associated with the laterstored segment update retrieved from the space reclamation sourcecartridge 136(s). Thus, when the entry 160(e) is established in thecartridge segment directory 141(m) for each segment update that isretrieved from the space reclamation source cartridge 136(s), the bit ofthe auxiliary segment bit map 143 associated with the segment update'ssegment identifier is checked, and if it is set the invalid bit 162 ofthe entry 160(e) is set. Accordingly, when the cartridge 136(m) is laterused as a space reclamation source cartridge 136(s'), the entry 160(e)will be deemed associated with an invalid segment update, and will beignored when candidate segment updates are selected as described above.

In one particular embodiment, the bits of the segment bit map 142(1)associated with the current logging cartridge 136(1) can be set as thevalid segment updates are stored on the cartridge 136(1), but thesegment bit maps 142(m) (m≠1) associated with the other cartridges136(m≠1) will be conditioned when the current logging cartridge is fulland unloaded from the drive and a new current logging cartridgeselected. In that case, the auxiliary segment bit map 143 will beinitialized by copying the contents of the segment bit map 142(1)associated with the current logging cartridge 136(1) at the beginning ofthe space reclamation operation. Thus, at the beginning of a spacereclamation operation, the auxiliary segment bit map 143 will identifyall of the segments for which valid segment updates were stored on thecartridge which comprises the current logging cartridge 136(1) at thebeginning of the space reclamation operation. As the space reclamationoperation proceeds, the bits of the auxiliary segment bit map 143 willalso identify all of the segments which are stored on the cartridge orcartridges (if multiple cartridges are used as logging cartridges duringthe space reclamation operation) which comprise current loggingcartridges during the space reclamation operation. Initializing theauxiliary segment bit map 143 in this manner insures that segmentupdates were stored on the current logging cartridge 136(1) prior to thebeginning of the space reclamation operation, which would not have beenreflected in the space reclamation source cartridge's segment bit map142(s), will be reflected in the auxiliary segment bit map 143.

The detailed operations performed by the tape log control module 135 inconnection with a storage operation and a space reclamation operationwill be described in connection with the flow charts depicted in FIGS.10 and 11. Generally, operations initiated by the tape log controlmodule 135 will be controlled by

(i)the number of empty tape cartridges falling below a predeterminedthreshold number, which will cause a space reclamation operation to beinitiated; and.

(ii) the input queue 132 having a head input queue element 132(1) whoserecords are to be stored on the respective tape log set 130.

The operations performed by the tape log control module 135 for each ofthese will be described in connection with FIGS. 10 and 11,respectively.

FIG. 10 depicts operations performed by the tape log control module 135when the number of empty tape cartridges falling below a predeterminedthreshold number. As indicated above, when the number of empty tapecartridges falls below the predetermined threshold number, a spacereclamation operation will be initiated to identify one of the tapecartridges 136(m) to be used as a space reclamation source cartridge136(s), identify ones of the segment updates recorded on the spacereclamation source cartridge 136(s) as valid candidates to be copied tothe tape cartridge 136(1) currently being used for logging, and retrievethe identified valid candidate segment updates and selectively generateinput queue elements 132(i) therefor and link the generated input queueelements to the input queue. As described above, the determination as towhether an input queue element 132(i) will be generated for a candidatesegment update which has been identified as valid is based on thecondition of the bit of the auxiliary segment map associated with thecandidate segment update's segment identifier, which if set indicatesthat a segment update received from the filter/buffer module 51 has beenstored on a tape cartridge 136(m) since the reconstruction operation hasbeen initiated.

With reference to FIG. 10, during a reclamation operation the tape logcontrol module 135 will initially perform a number of steps to identifyone of the tape cartridges 136(m) to be used as a space reclamationsource cartridge 136(s), including:

(i) determine whether the number of empty tape cartridges 136(m) hasfallen below a predetermined threshold value (step 310);

(ii) in response to a positive determination in step 310, identify foreach tape cartridge 136(m) the number of bits of the segment bit map142(m) which are set (step 311);

(iii) identify, among tape cartridges for which the number determined instep 311 is other than zero, the tape cartridge 136(m) which has theleast number of set bits of the segment bit map as determined in step311(step 312), and

(iv) initialize an auxiliary segment bit map 143 (step 313).

Since the segment bit map 142(m) associated with a tape cartridge 136(m)identifies each segment for which a valid segment update is recorded onthe tape cartridge 136(m), the tape cartridge 136(m) associated with thesegment bit map 142(m) which has the least number of bits set also hasthe least number of valid segment updates recorded on any of the tapecartridges in the tape log set. Thus, the tape cartridge identified instep 312 will contain the least number of valid segment updates, and isselected as the space reclamation source cartridge.

Following step 313, the tape log control module 135 performs a number ofsteps to identify ones of the segment updates recorded on the spacereclamation source cartridge 136(s) as valid candidates to be copied tothe tape cartridge 136(1) currently being used for logging. As describedabove, the cartridge segment directory 141(s) and segment bit map 142(s)associated with the space reclamation source cartridge 136(s) are usedto identify the segment updates which are to be candidates for copying,with the segment directory 141(s) being scanned in reverse order and thevalid and invalid bits 161 and 162 being selectively conditioned. At theend of these steps, the segment updates associated with entries 160(e)for which the valid bits 161 are set comprise the valid candidates forcopying. More specifically, during those operations the tape log controlmodule 135 will:

(i) select the last entry 160(E) of the cartridge segment directory141(s) associated with the space reclamation source cartridge 136(s)(step 320);

(ii) determine whether the invalid bit 162 of the selected entry is set(step 321)

(iii) in response to a positive determination in step 321, determinewhether there is a previous entry in the cartridge segment directory141(s) (step 322), and, if it makes a positive determination return tostep 321 to process the previous entry.

(iv) Returning to step 321, if the tape log control module 135determines that the invalid flag of the selected entry is not set, ittests the bit of the segment bit map 142(m) associated with the segmentidentifier contained in field 163 of the selected entry to determinewhether that bit is set (step 323;

(v) in response to a negative determination in step 323, the tape logcontrol module 135 sets the invalid bit 162 of the selected entry of thecartridge record directory 141(m) (step 324); but

(vi) in response to a positive determination in step 323, the tape logcontrol module 135 sets the valid bit 161 of the selected entry of thecartridge record directory 141(m) (step 325), and

(vii) clears the bit of the bit of the segment bit map 142(m) associatedwith the segment identifier contained in field 163 of the selected entry(step 326).

Thus, steps 321 and 322 will ensure that, if the invalid bit associatedwith an entry 160(e) is set prior to beginning the operations describedabove in connection with steps 320 through 326, the valid bit for theentry, which identifies the valid candidates for copying, will not beset. On the other hand, if the invalid bit is not set prior to beginningthe operations described above in connection with steps 320 through 326,the valid bit will be set only if the bit of the space reclamationsource cartridge's segment bit map 142(s) that is associated with thesegment update's segment identifier is set. Step 326 will ensure thatthis occurs for only one such entry 160(e), namely, the last entry inthe cartridge segment directory 141(m) which contains the segmentidentifier for which the invalid flag is not set. Following either step324 or step 326, the tape log control module 135 returns to step 322 todetermine whether there is a previous entry in the cartridge segmentdirectory 141(s) associated with the space reclamation source cartridge136(s) and, if so, returns to step 321 to begin processing that entry.

The tape log control module 135 performs the operations described abovein connection with steps 320 through 326 through one or more iterationsuntil it determines in step 322 that there are no more entries in thecartridge segment directory 141(s) associated with the space reclamationsource cartridge. At that point, the tape log control module 135proceeds to a series of steps to retrieve the valid candidate segmentupdates which have been identified and selectively generate input queueelements 132(i) therefor and link the generated input queue elements tothe input queue. In those operations, the tape log control module 135will:

(i) enable the identified space reclamation source cartridge 136(s) tobe loaded into a drive and the first segment update retrieved therefrom(step 330);

(ii) determine whether the entry 160(e) of the cartridge segmentdirectory 141(s) associated with the retrieved segment update contains avalid bit 161 that is set (step 331);

(iii) in response to a negative determination in step 331, determinewhether there is a subsequent segment update recorded on the spacereclamation source cartridge 136(s) (step 332)

(iv) in response to a positive determination in step 332, enable thenext segment update to be retrieved from the space reclamation sourcecartridge 136(s) (step 333) and return to step 331 to initiateprocessing for that segment update.

(v) In response to a positive determination in step 331, test the bit ofthe auxiliary segment bit map 143 associated with the segment identifierwhich is associated with the retrieved segment update to determinewhether it is set (step 334).

If there is a positive determination in step 334, the tape log controlmodule 135 returns to step 332, which will cause the most recentlyretrieved segment update to be ignored. As described above, if the bitof the auxiliary segment bit map 143 is set, a segment update having thesame segment identifier and received from the filter/buffer module 51has been stored on a tape cartridge 136(m) during the space reclamationoperation, and so the segment update received from the space reclamationsource cartridge will be ignored. Accordingly, if there is a positivedetermination in step 334, the tape log control module 135 returns tostep 332 to determine whether there is a subsequent segment updaterecorded on the space reclamation source cartridge 136(s) and, if so,initiate processing of that segment update.

Returning to step 334, if the tape log control module 135 makes anegative determination in that step, no segment update having the samesegment identifier and received from the filter/buffer module 51 hasbeen stored on a tape cartridge 136(m) during the space reclamationoperation. In that case, the tape log control module 135 generates aninput queue element 132(i) for the segment update and links it to theinput queue 132, in a manner similar to that described above inconnection with generation of an input queue element 132(i) for segmentupdate information received from the filter buffer module (FIG. 9A)(step 335), except that the input queue element 132(i) generated in step335 indicates that the information contained therein originated from thespace reclamation source module. Thereafter, the tape log control module135 returns to step 332 to determine whether there is a subsequentsegment update recorded on the space reclamation source cartridge and,if so, initiate processing of that segment update.

Returning to step 332, if the tape log control module 135 determines inthat step that the space reclamation source cartridge 136(s) contains nofurther segment updates to be processed, all of the possible segmentupdates which are candidates for copying from the space reclamationsource cartridge 136(s) onto the current logging cartridge 136(1) havebeen retrieved from the space reclamation source cartridge, and so allof the bits of the segment bit map 142(s) associated with the spacereclamation source cartridge 136(s) can be reset (step 336), indicatingthat it is empty.

FIG. 11 depicts operations performed by the tape log control module 135when the input queue 132 has a head input queue element 132(1) whosesegment update information is to be stored on the respective tape logset 130. It will be appreciated that the input queue 132 will have ahead input queue element if it contains at least one input queueelement. With reference to FIG. 11, during those operations the tape logcontrol module 135:

(i) determines whether the input queue 132 has a head input queueelement 132(1) (step 340);

(ii) in response to a positive determination in step 340, determineswhether the current logging cartridge 136(1) contains enough space tostore the segment update information contained in the head input queueelement 132(1) (step 341);

(iii) in response to a positive determination in step 341, enables thesegment update information from the head input queue element 132(1) tobe stored on the current logging cartridge 136(1) (step 342);

(iv) establishes an entry 160(e) in the cartridge segment directory141(1) associated with the current logging cartridge for the storedsegment update information, in the process storing the segment update'ssegment identifier in field 163 of the new entry 160(e) (step 343); and

(v) determines whether the segment update information contained in thehead input queue element 132(1) was received from the filter/buffermodule 51(step 344).

(vi) If the tape log control module 135 makes a negative determinationin step 344, the segment update information from the head input queueelement 132(1) recorded on the current logging tape cartridge 136(1) hadbeen received from the space reclamation source cartridge during a spacereclamation operation, and so the tape log control module 135 will testthe auxiliary segment bit map 143 to determine whether the bitassociated with the segment update's segment identifier is set (step345), and

(vii) in response to a positive determination in step 344, set theinvalid bit of the entry 160(e) established in step 342 (step 346).

On the other hand, if the tape log control module 135 makes a negativedetermination in step 345 (which will occur if the bit in the auxiliarysegment bit map 143 associated with the segment update's segmentidentifier is not set), or if it makes a positive determination in step344 (which will occur if the segment originated from the filter/buffermodule 51), the tape log control module 135 will set the bit associatedwith the segment update's segment identifier in the segment bit map142(1) associated with the current logging tape cartridge 136(1) (step347), thereby to indicate that the current logging tape cartridge 136(1)contains the most recently received segment update for the segment.Thereafter, the tape log control module can dequeue the head input queueelement 132(1) from the input queue 132 and enqueue it to the tape logmodule's queue element source for use later in establishing input queueelements (step 348), thereby to establish the next input queue elementas the head input queue element.

Returning to step 341, if the tape log control module 135 makes anegative determination in that step, that is, if it determines that thecurrent logging tape cartridge 136(1) does not have enough space for theinformation to be stored in the head input queue element 132(1), it willproceed to a series of steps to unload the current logging cartridgefrom the drive, replace it with a new current logging cartridge, andupdate various data structures. In particular, the tape log controlmodule 135 will:

(i) unload the current logging cartridge 136(1) from the drive (step350);

(ii) reset the various bits of the segment bit maps 142(m) (m≠l) whichare associated with the other tape cartridges 136(m) (m≠1) whichcorrespond to the bits that are set in the segment bit map 142(1) whichis associated with the cartridge that was unloaded in step 350 (step351);

(iii) select an empty tape cartridge 136(m) from the tape logging set130 and load it into the drive as the new current logging cartridge136(1') (step 352); and

(iv) initialize the cartridge segment directory 141(1') and segment bitmap 142(1') for the new current logging cartridge (step 353).

Thereafter, the tape log control module 135 returns to step 342 toperform the operations described above to store the information from thehead input queue element 132(1) on the new current logging cartridge136(1'). It will be appreciated that the tape log control module 135will enable the operations described above in connection with steps 340through 348 and 350 through 353 as long as the input queue 132 is notempty, in connection with successive head input queue elements.

E. Reconstruction Module 53

The reconstruction module 53 performs a reconstruction operation tofully or partially reconstruct the information stored on a singlestorage device 22 (FIG. 2), multiple storage devices or an entire massstorage subsystem 14(n) (FIG. 1). A full reconstruction may benecessitated by, for example, a catastrophic failure in connection withthe protected volume, in which case the remote data protection facility5 may supply the reconstructed information to, for example, the originaldigital data processing system 10(n) or another digital data processingsystem 10(n') (n'≠n) if the original digital data processing system10(n) is unavailable. On the other hand, a partial reconstruction may benecessitated by, for example, a failure of one or a limited number ofstorage devices 22 (FIG. 2) on the original digital data processingsystem 10(n), and the reconstructed information may be reconstructed toother storage devices on the same digital data processing system 10(n).

In performing a reconstruction operation, the reconstruction module 53will retrieve the most recently-stored segment updates for the protectedvolume(s) whose data is (are) being reconstructed from the tapecartridges 136(m) of the tape log set 130 that is associated with theprotected volume(s), in the process using the cartridge segmentdirectories 141(m) and segment bit maps 142(m) that are associated withthose cartridges 136(m) to identify those segment updates. In addition,the reconstruction module will use the block queue 70 and segmentidentifier hash table 71 of the filter/buffer module 51 to identify themost recently-received segment updates which have not been stored on thetape log set 130. A reconstruction operation effectively proceeds inseveral phases, as generally illustrated in the flow chart in FIG. 12,including

(i) a segment update validation phase (step 400), in which entries160(e) of the cartridge record directories 141(m) are validated toidentify the most recently-stored valid ones of the segment updates forthe protected volume(s) whose data is (are) being reconstructed that isrecorded on the tape log set,

(ii) retrieving the segment updates which have been validated from thecartridges of the tape log set (step 401),

(iii) using the segment identifier hash table 71 to identify ones of theblock queue elements 75(b) of the block queue whose blocks 77 containthe most recently-received segment updates for the protected volume(s)whose data is (are) being reconstructed (step 402),

(iv) retrieving the segment updates associated with the protectedvolume(s) whose data is (are) being reconstructed from the blocksidentified in step 402 (step 403),

(v) merging the ones of the segment updates retrieved from the blockqueue 70 in step 403 that comprise partial segment updates, into thefull segment updates retrieved from either the tape log set in step 401or from the block queue 70 in step 403 (step 404), thereby to generatereconstructed segment updates, and

(vi) providing the reconstructed segment updates to the output module 54for transfer to the mass storage subsystem 14(n) to receive thereconstructed information (step 405).

Operations performed by the reconstruction module 53 in connection withthe segment update validation step (step 401) are generally similar tothose performed by the tape log control module 135 in validating segmentupdates during a space reclamation operation, as described above inconnection with FIG. 10, except that the reconstruction module 53 needonly validate the segment updates for segments that are associated withthe protected volume(s) whose data is being reconstructed.

In addition, in identifying the ones of the block queue elements 75(b)whose blocks 77 contain the most recently-received segment updates forthe protected volume(s) whose data is (are) being reconstructed (step402), the reconstruction module 53 can scan through all of the lists111(h) to locate entries whose segment identifier fields 112 correspondto the segment identifiers for the protected volume(s) whose data is(are) being reconstructed.

Although the reconstruction module 53 has been described as merging thesegment updates retrieved from the block queue 70 into the segmentupdates retrieved from the tape log set 130 in step 404, it will beappreciated that the merging operation can alternatively be performed bythe mass storage subsystem 14(n) which is to receive the reconstructedinformation.

F. Output Module 54

FIG. 7 depicts the structure of the output module 54 useful in theremote data protection facility 5. With reference to FIG. 7, the inputmodule 50 includes a plurality of interfaces 180(1) through 180(N)(generally identified by reference numeral 180(n)) each of which isconnected to transmit information received from the tape log module 52(FIG. 6) to a correspondingly-indexed digital data processing system10(n) over a communication link 12(n). Each interface 180(n) receivesdigital information from the tape log module 52 from a particular tapelog set 130 and generates in response thereto signals, either inelectrical or optical form, for transmission to the associated digitaldata processing system 10(n).

III. Summary

The remote data protection facility 5 described above provides a numberof advantages. The remote data protection facility 5 provides anarrangement which can provide efficient remote mirrored data storage forone or more digital data processing systems 10(n), which can protectagainst loss of digital data for any of a number of reasons. The remotedata protection facility 5 and the remote data protection facilityinterface 17 (FIG. 2) cooperate to enable segment updates to betransferred from the respective mass storage subsystem 14(n) to theremote data protection facility 5, and to enable reconstructed segmentsto be transferred from the remote data protection facility 5 to therespective mass storage subsystem 14(n), without requiring assistance ofor control by a host computer 11(n). Thus, the mirrored data storage iseffectively transparent to the host computers 11(n) and their operators.

The buffering and filtering performed by the filter/buffer module 51provides for the efficient storage of segment updates that are receivedfrom the digital data processing systems 10(n), and serves to reduce thenumber of segment updates which are logged by the tape log module 52. Inmany data processing applications, retrievals from and updates to dataare relatively localized, that is, the applications may process andupdate data in the same segment several times within a relatively shortperiod of time. The buffering and filtering performed by thefilter/buffer module 51 will assist in ensuring that all of the updates(which will primarily comprise partial segments) will be buffered by theremote data protection facility 5, but that segments (which in oneembodiment comprise full segments) will not be provided to the tape logmodule 52 for logging until some time has passed since the applicationsbegan processing data from a segment. Thus, if the application isprocessing in a localized manner, the segments provided to the tape logmodule 52 for logging will preferably contain at least some substantialportion of the localized processing for each respective segment for atleast some period of time.

In addition, since the filter/buffer module 51 stores the segmentupdates in a queue, namely, the block queue 70, the ordering of thereceipt of segment updates for each segment that are buffered by thefilter/buffer module 51 can be readily determined from their relativepositions in the queue.

The tape log module 52 also provides a number of advantages. By dividingthe cartridges which are provided by the tape log module 52 into aplurality of log sets, and providing that segment updates from eachprotected volume are logged on one of the tape log sets, instead of anyof the tape cartridges, when information stored on a protected value isto be reconstructed, the tape log module 52 effectively reduces thenumber of cartridges that need to be processed to retrieve theinformation required for the reconstruction, and thus reduce the timewhich is necessary for a reconstruction.

Furthermore, the various data structures established and maintained bythe tape log module 52, in particular the cartridge segment directories141(m), the segment bit maps 142(m) and the auxiliary segment bit map143, provide an efficient mechanism for rapidly identifying the mostrecently-logged segment updates stored in the tape log module 52. Foreach tape cartridge, the segment bit maps 142(m) efficiently identifythe valid segment updates which are stored on the respective cartridge,but not necessarily which segment updates on the cartridge are valid.The cartridge segment directories 141(m), on the other hand, identifythe series of segment updates which are recorded on each respective tapecartridge. Since, except for segment updates provided by during a spacereclamation operation, the segment updates for each segment are storedon the tape cartridges in the order they are received, the valid segmentupdate for each segment as received from the filter/buffer module 51 canbe efficiently determined as the last segment update on the tapecartridge as indicated by the cartridge's cartridge segment directory141(m). During a space reclamation operation, this ordering may not bepreserved, and the auxiliary segment bit map 143 is provided to assistin invalidating segment updates for which the ordering is not preserved.

In addition, the reconstruction module 53 provides for the efficientreconstruction of information for the protected volumes using the datastructures which are established and maintained by the filter/buffermodule 51 and the tape log module 52 for their purposes, and withoutrequiring additional structures to support reconstruction.

It will be appreciated that a number of modifications may be made to theremote data protection facility 5. For example, while the remote dataprotection facility 5 has been described as providing back-up mirroredstorage of digital data from a number of digital data processing systems10(n), it will be appreciated that the remote data protection facility 5can provide back-up mirrored storage for a single digital dataprocessing system. In addition, the remote data protection facility 5may provide storage for digital data that may be provided by, and usedby, any source or destination of digital data, including, for example,video data in digital form.

Furthermore, although the invention has been described as providing aremote data protection facility 5 and mass storage subsystem 14(n) whichprovides for back-up mirrored storage of digital data without therequirement of a host computer 11(n) controlling the transfer of data tothe remote data protection facility 5 for back-up mirrored storage, orthe receiving reconstructed data from the remote data protectionfacility 5, it will be appreciated that such data transfers between themass storage subsystem 14(n) and remote data protection facility 5 mayproceed under control of one or more host computers 11(n) which may beprovided in a digital data processing system 10(n). In addition, it willbe appreciated that, for a particular remote data protection facility 5,data transfers with one or more of the digital data processing systemsmay be controlled by a host computer, whereas data transfers with one ormore other digital data processing systems may proceed independently ofcontrol by a host computer.

In addition, although one embodiment of the remote data protectionfacility 5 has been described as including an autochanger which provideseighty-eight digital linear tape (DLT) cartridges divided into eight logsets, and nine drives, the remote data protection facility 5 mayadvantageously make use of such storage elements having more or fewercartridges, log sets and drives. Preferably, at least one more drivewill be provided than the number of log sets, so that space reclamationwith a cartridge from at least one log set can proceed contemporaneouslywith data storage for all of the log sets which may be provided in theremote data protection facility. In addition, although a relativelynon-random access storage medium such as tape is advantageously used forthe logging operations performed by the remote data protection facility5, storage media other than or in addition to tape may be used.

Further, although in one embodiment the filter/buffer module 51 has beendescribed as buffering information received thereby using one or moredisk storage devices, it will be appreciated that other digital datastorage devices, such as conventional random access memories, may beused in instead of the disk storage devices or to augment the storageprovided by the disk storage devices.

In addition, although in one embodiment the filter/buffer module 51 hasbeen described as providing a hash table 71 for storing pointers toblocks in the block queue which contain valid segment updates, it willbe appreciated that other arrangements may be used. For example, a listmay be provided having one entry for each possible segment identifier,in which each entry for which a valid segment update is being bufferedin the filter/buffer module 51 contains a pointer to the block queueentry 75(b) which contains the valid segment update. Depending on thepercentage of segments for which segment updates are buffered in thefilter/buffer module 51, it will be appreciated that such a list may besparsely populated, but using such a list instead of a hash table 71 canreduce the time required to identify the particular block queue entry75(b) which contains the valid segment update since no hashing functionwould need to be computed. If a hash table 71 is used, any convenienthashing function can be selected for use in determining the particularlinked list 111(h) associated with each segment identifier.

Furthermore, although the filter/buffer module 51 has been described asmaintaining the hash table 71 so as to point to the block 77 whichcontains only the most recently-received segment update for each segmentidentifier, it will be appreciated that the filter/buffer module 51 mayinstead maintain the hash table so as to point to multiple segmentupdates. This can be particularly advantageous in connection withpartial segments, and particularly when the partial segments aredisjoint (that is, nonoverlapping) or partially overlapping, since eachpartial segment may contain updated information which may not be updatedin others of the partial segments for the same segment identifier.Although in one embodiment the partial segments are not transferred tothe tape log module 52 for storage, if the most recent segment update(s)received for a particular segment is a (are) partial segment(s), thepartial segment(s) can be used by the reconstruction module 53 inreconstructing the segment. In such a reconstruction operation, thereconstruction module 53 can reconstruct the segment using the mostrecently received full segment update, which may be stored in the tapelog module 52 or in a block queue element 75(b), and merging therein thepartial segment(s) which were received after the most recently receivedfull segment update. The reconstruction module 53 can use the pointer(s)in the hash table 71 to the various segment update(s) for the segment inthe block queue 70 to identify the ones of the block queue elements75(b) which contain valid segment update(s) for use in thereconstruction operation. It will be appreciated that, when thefilter/buffer module 51 receives a full segment update for a particularsegment, it will invalidate all previously-received partial segmentupdates for the segment as indicated in the hash table 71.

In addition, although the filter/buffer module 51 has been described inone embodiment as maintaining the segment updates that it is bufferingin a queue, it will be appreciated that other arrangements may be usedto store the segment updates. Similarly, although the tape log module 52has been described as maintaining the segment updates to be stored onthe current logging cartridge 136(1) in an input queue 132, it will beappreciated that other arrangements may be used to store such segmentupdates. Furthermore, although the remote data protection facility 10has been described as providing storage for fixed-length segments, itwill be appreciated that the facility 10 may instead or also be used toprovide storage for variable length structures such as records.

It will be appreciated that a system in accordance with the inventioncan be constructed in whole or in part from special purpose hardware ora general purpose computer system, or any combination thereof, anyportion of which may be controlled by a suitable program.

The foregoing description has been limited to a specific embodiment ofthis invention. It will be apparent, however, that various variationsand modifications may be made to the invention, with the attainment ofsome or all of the advantages of the invention. It is the object of theappended claims to cover these and such other variations andmodifications as come within the true spirit and scope of the invention.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A valid data item update retrieval and storagesubsystem for copying valid ones of a plurality of data item updatesserially recorded on a source storage medium, onto a target storagemedium, each data item update having a data item identifier which is oneof a set of data item identifier values, the valid data item updateretrieval and recording arrangement comprising:A. a storage mediumdirectory including a series of directory entries each identifying, fora corresponding one of the series of data item updates recorded on saidsource storage medium, the data item identifier associated with saidcorresponding one of the series of data item updates; B. a data itemidentifier flag set comprising a plurality of data item identifierflags, each associated with a data item identifier, each flag having avalid condition indicating that the source storage medium has a validdata item update associated with the respective data item identifierflag's associated data item identifier stored thereon, and at least oneother condition; C. a valid data item update identifier for using thestorage medium directory and said data item identifier flag set toidentify a set of valid data item updates on said source storage medium,the valid data item update identifier scanning the directory entries insaid directory in reverse in reverse order and, for each directoryentry, determining whether the data item identifier flag associated withthe data item identifier contained in the directory entry indicates thatthe source cartridge has a valid data item update associated with therespective flag's associated data item identifier stored thereon and, ifso, determining that the data item update associated with the directoryentry is a valid data item update; D. a valid data item update transfercontrol for providing valid ones of the data item updates as identifiedby said valid data item update identifier from the source storage mediumfor storage on the target storage medium.
 2. A valid data item updateretrieval and storage subsystem as defined in claim 1 in which the validdata item update identifier further condition the one of the data itemidentifier flags associated with the data item identifier to said atleast one other condition when it determines that the data item updateassociated with the directory entry is a valid data item update.
 3. Avalid data item update retrieval and storage subsystem as defined inclaim 2 in which said valid data item update identifier includes:A. adirectory entry selector for selecting a directory entry; B. a validitydetermination generation element for using the data item identifier flagassociated with the data item identifier identified by the selecteddirectory entry generate a validity indication indicating whether theselected directory entry is associated with a valid data item update; C.a data item identifier flag conditioner responsive to the validityindication generated by the validity determination generation elementindicating that the selected directory entry is associated with a validdata item update for conditioning the data item identifier flagassociated with the data item identifier identified by the selecteddirectory entry to said at least one other condition; and D. aniteration control element for controlling the directory entry selectorelement, said validity determination generation element and said dataitem identifier flag conditioner through a series of iterations.
 4. Avalid data item update retrieval and storage subsystem as defined inclaim 3 in which said iteration control element enables said directoryentry selector to, in each of a series of iterations, select the one ofthe directory entries which precedes the directory entry selected duringthe preceding iteration.
 5. A valid data item update retrieval andstorage subsystem as defined in claim 4 in which said iteration controlelement enables said directory entry selector to, in a first iteration,select the last directory entry in the storage medium directory.
 6. Avalid data item update retrieval and storage subsystem as defined inclaim 3 in which said iteration control element terminates iterationsafter the iteration in which the directory entry selector selects thefirst directory entry in the storage medium directory.
 7. A valid dataitem update retrieval and storage subsystem as defined in claim 1further comprising a data item update storage control for controllingstorage of valid data item updates provided by said valid data itemupdate transfer control on the target storage medium.
 8. A valid dataitem update retrieval and storage subsystem as defined in claim 7 inwhich said data item update storage control further controls storage ofdata item updates provided by a data item update source on the targetstorage medium.
 9. A valid data item update retrieval and storagesubsystem as defined in claim 8, the target storage medium havingassociated therewith a target storage medium data item identifier flagset comprising a plurality of target storage medium data item identifierflags, each associated with a data item identifier, the data item updatestorage control conditioning the target storage medium data itemidentifier flags in response to the storage of data item updates on thetarget storage medium.
 10. A valid data item update retrieval andstorage subsystem as defined in claim 9 in which each target storagemedium data item identifier flag has having a valid condition and atleast one other condition, the data item update storage controlconditioning the target storage medium data item identifier flagsassociated with data item identifiers for data item updates receivedfrom the data item update source and stored on said target storagemedium to the valid condition.
 11. A valid data item update retrievaland storage subsystem as defined in claim 10, the valid data item updateretrieval and storage subsystem being used in a system comprising atleast one other storage medium having associated therewith a data itemidentifier flag set comprising a plurality of data item identifierflags, each associated with a data item identifier, each data itemidentifier flag having a valid condition indicating that the otherstorage medium has a valid data item update associated with therespective data item identifier flag's associated data item identifierstored thereon, and at least one other condition, the data item updatestorage control further conditioning the data item identifier flag ofthe data item identifier flag set associated with the other storagemedium to said other condition for each target storage medium data itemidentifier flag which has the valid condition.
 12. A valid data itemupdate retrieval and storage subsystem as defined in claim 9 in whicheach target storage medium data item identifier flag has has a validcondition and at least one other condition, the data item update storagecontrol conditioning the target storage medium data item identifierflags associated with data item identifiers for data item updatesreceived from the data item update source and stored on said targetstorage medium to the valid condition.
 13. A valid data item updateretrieval and storage subsystem as defined in claim 12 in which saiddata item update storage control further selectively conditions thetarget storage medium data item identifier flags associated with dataitem identifiers for data item updates received from the source storagemedium to one of the valid condition or the other condition.
 14. A validdata item update retrieval and storage subsystem as defined in claim 13,the data item update storage control conditioning the target storagemedium data item identifier flags associated with data item identifiersfor valid data item updates received from the source storage medium tothe valid condition.
 15. A valid data item update retrieval and storagesubsystem as defined in claim 1, further comprising a data item updatestorage control for controlling storage of successive data item updateson said target storage medium, the data item update storage controlmaintaining a storage medium directory for said target storage medium,the data item update storage control providing an entry for each dataitem update stored on said trget storage medium including the data itemidentifier, so that the successive entries in the target storagemedium's storage medium directory identify the successive data itemidentifiers associated with successive data item updates stored on thetarget storage medium.
 16. A valid data item update retrieval andstorage subsystem as defined in claim 15 in which each entry of saidtarget storage medium's storage medium directory further includes aninvalid flag having an invalid conditin and at least one othercondition, the condition of the invalid flag being controlled by saiddata item update storage control.
 17. A valid data item update retrievaland storage subsystem as defined in claim 16, the valid data item updateidentifier and the valid data item update transfer control operatingduring a space reclamation operation in which valid data item updatesare copied from the source storage medium to the target medium, the dataitem update storage control during the space reclamation operationmaintaining an auxiliary data item identifier flag set comprising aplurality of auxiliary data item identifier flags, each associated witha data item identifier, each flag having a plurality of conditions, thedata item update storage control controlling the condition of theinvalid flag of selected ones of the entries of said target storagemedium's storage medium directory in relation to the condition of theauxiliary data item identifier flag asociated with the data itemidentifier of the respective entries.
 18. A valid data item updateretrieval and storage subsystem as defined in claim 17, in which saiddata item update storage control further controls storage of data itemupdates provided by a data item update source, each being associatedwith a said data item identifier, on the target storage medium, the dataitem update storage control including:A. an auxiliary data itemidentifier flag conditioner for, during a said space reclamationoperation, conditioning the auxiliary data item identifier flagassocaited with the data item identifier of each data item updateprovided by said data item update source to a predetermined condition;and B. a target storage medium directory entry generator for generatingan entry for the target storage medium's storage medium directory, thetarget storage mdeium directory entry generator conditioning the entry'sinvalid flag to an invalid condition if the data item update stored onthe target storage medium was provided by the valid data item updatetransfer control and the auxiliary data item identifier flag associatedwith the data item identifier of the data item update had predeterminedcondition.
 19. A method of copying valid ones of a plurality of dataitem updates serially recorded on a source storage medium, onto a targetstorage medium, each data item update having a data item identifierwhich is one of a set of data item identifier values, a set of saidvalid data item updates for storing on a valid data item update storagemedium, the valid data item update retrieval and recording arrangementcomprising:A. providing a storage medium directory including a series ofdirectory entries each identifying, for a corresponding one of theseries of data item updates recorded on said source storage medium, thedata item identifier associated with said corresponding one of theseries of data item updates; B. providing a data item identifier flagset comprising a plurality of data item identifier flags, eachassociated with a data item identifier, each flag having a validcondition indicating that the source storage medium has a valid dataitem update associated with the respective data item identifier flag'sassociated data item identifier stored thereon, and at least one othercondition; C. for using the storage medium directory and said data itemidentifier flag set to identify a set of valid data item updates on saidsource storage medium, the valid data item update identifier scanningthe directory entries in said directory in reverse in reverse order and,for each directory entry, determining whether the data item identifierflag associated with the data item identifier contained in the directoryentry indicates that the source cartridge has a valid data item updateassociated with the respective flag's associated data item identifierstored thereon and, if so, determining that the data item updateassociated with the directory entry is a valid data item update; D.providing valid ones of the data item updates as identified by saidvalid data item update identifier from the source storage medium forstorage on the target storage medium.
 20. A method as defined in claim19 in which the valid data item update identification step furtherincludes the step of conditioning the one of the data item identifierflags associated with the data item identifier to said at least oneother condition when it determines that the data item update associatedwith the directory entry is a valid data item update.
 21. A method asdefined in claim 20 in which said valid data item update identificationstep includes the steps of iteratively:A. selecting a directory entry;B. using the data item identifier flag associated with the data itemidentifier identified by the selected directory entry generate avalidity indication indicating whether the selected directory entry isassociated with a valid data item update; and C. conditioning the dataitem identifier flag associated with the data item identifier identifiedby the selected directory entry to said at least one other condition inresponse to the validity indication indicating that the selecteddirectory entry is associated with a valid data item update.
 22. Amethod as defined in claim 21 in which, during the directory entryselection step, in each iteration the one of the directory entries isselected which precedes the directory entry selected during thepreceding iteration.
 23. A method as defined in claim 22 in which,during the directory entry selection step of a first iteration, the lastdirectory entry in the storage medium directory is selected.
 24. Amethod as defined in claim 21 in which iterations are terminated afterthe iteration in which the first directory entry in the storage mediumdirectory is selected during the directory entry selection step.
 25. Amethod as defined in claim 19 further comprising the step of controllingstorage of valid data item updates provided by said valid data itemupdate transfer control on the target storage medium.
 26. A method asdefined in claim 25 in which, during the data item update storage step,data item updates provided by a data item update source are also storedon the target storage medium.
 27. A method as defined in claim 26, thetarget storage medium having associated therewith a target storagemedium data item identifier flag set comprising a plurality of targetstorage medium data item identifier flags, each associated with a dataitem identifier, the data item update storage step including the step ofconditioning the target storage medium data item identifier flags inresponse to the storage of data item updates on the target storagemedium.
 28. A method as defined in claim 27 in which each target storagemedium data item identifier flag has having a valid condition and atleast one other condition, the data item update storage step includingthe step of conditioning the target storage medium data item identifierflags associated with data item identifiers for data item updatesreceived from the data item update source and stored on said targetstorage medium to the valid condition.
 29. A method as defined in claim28, the method being used in a system comprising at least one otherstorage medium having associated therewith a data item identifier flagset comprising a plurality of data item identifier flags, eachassociated with a data item identifier, each data item identifier flaghaving a valid condition indicating that the other storage medium has avalid data item update associated with the respective data itemidentifier flag's associated data item identifier stored thereon, and atleast one other condition, the data item update storage step furtherincluding the step of conditioning the data item identifier flag of thedata item identifier flag set associated with the other storage mediumto said other condition for each target storage medium data itemidentifier flag which has the valid condition.
 30. A method as definedin claim 27 in which each target storage medium data item identifierflag has has a valid condition and at least one other condition, thedata item update storage step further including the step of conditioningthe the target storage medium data item identifier flags associated withdata item identifiers for data item updates received from the data itemupdate source and stored on said target storage medium to the validcondition.
 31. A method as defined in claim 30 in which said data itemupdate storage step further includes the step of selectivelyconditioning the target storage medium data item identifier flagsassociated with data item identifiers for data item updates receivedfrom the source storage medium to one of the valid condition or theother condition.
 32. A method as defined in claim 31, the data itemupdate storage step including the step of conditioning the targetstorage medium data item identifier flags associated with data itemidentifiers for valid data item updates received from the source storagemedium to the valid condition.
 33. A method as defined in claim 19,further comprising the step of storing successive data item updates onsaid target storage medium, the data item update storage stepmaintaining a storage medium directory for said target storage medium,the data item update storage step including the step of providing anentry for each data item update stored on said target storage mediumincluding the data item identifier, so that the successive entries inthe target storage medium's storage medium directory identify thesuccessive data item identifiers associated with successive data itemupdates stored on the target storage medium.
 34. A method as defined inclaim 33 in which each entry of said target storage medium's storagemedium directory further includes an invalid flag having an invalidcondition and at least one other condition, the data item update storagestep including the step of conditioning the invalid flag.
 35. A methodas defined in claim 34, the valid data item update identification stepand the valid data item update transfer step being performed during aspace reclamation operation in which valid data item updates are copiedfrom the source storage medium to the target storage medium, the dataitem update storage step including the step of, during the spacereclamation operation, maintaining an auxiliary data item identifierflag set comprising a plurality of auxiliary data item identifier flags,each associated with a data item identifier, each flag having aplurality of conditions, the data item update storage step including thestep of controlling the condition of the invalid flag of selected onesof the entries of said target storage medium's storage medium directoryin relation to the condition of the auxiliary data item identifier flagassociated with the data item identifier of the respective entries. 36.A method as defined in claim 35, in which said data item update storagestep further includes the step of storing of data item updates providedby a data item update source, each being associated with a said dataitem identifier, on the target storage medium, the data item updatestorage step further including the steps of:A. during a said spacereclamation operation, conditioning the auxiliary data item identifierflag associated with the data item identifier of each data item updateprovided by said data item update source to a predetermined condition;and B. generating an entry for the target storage medium's storagemedium directory, the target storage medium directory entry generatorconditioning the entry's invalid flag to an invalid condition if thedata item update stored on the target storage medium was provided by thevalid data item update transfer control and the auxiliary data itemidentifier flag associated with the data item identifier of the dataitem update had predetermined condition.
 37. A valid data item updateretrieval and storage subsystem for copying valid ones of a plurality ofdata item updates serially recorded on a source storage medium, onto atarget storage medium, each data item update having a data itemidentifier which is one of a set of data item identifier values, a setof said valid data item updates for storing on a valid data item updatestorage medium, the valid data item update retrieval and recordingarrangement comprising:A. a digital data processor; and B. a controlsubsystem for controlling the processor, the control subsystemcomprising:i. a storage medium directory including a series of directoryentries each identifying, for a corresponding one of the series of dataitem updates recorded on said source storage medium, the data itemidentifier associated with said corresponding one of the series of dataitem updates; ii. a data item identifier flag set module for enablingthe processor to maintain a data item identifier flag set comprising aplurality of data item identifier flags, each associated with a dataitem identifier, each flag having a valid condition indicating that thesource storage medium has a valid data item update associated with therespective data item identifier flag's associated data item identifierstored thereon, and at least one other condition; iii. a valid data itemupdate identifier use the storage medium directory and said data itemidentifier flag set to identify a set of valid data item updates on saidsource storage medium, the valid data item update identifier scanningthe directory entries in said directory in reverse in reverse order and,for each directory entry, determining whether the data item identifierflag associated with the data item identifier contained in the directoryentry indicates that the source cartridge has a valid data item updateassociated with the respective flag's associated data item identifierstored thereon and, if so, determining that the data item updateassociated with the directory entry is a valid data item update; iv. avalid data item update transfer control for enabling the processor toprovide valid ones of the data item updates as identified by said validdata item update identifier from the source storage medium for storageon the target storage medium.
 38. A valid data item update retrieval andstorage subsystem as defined in claim 37 in which the valid data itemupdate identifier module further enables the processor to condition theone of the data item identifier flags associated with the data itemidentifier to said at least one other condition when it determines thatthe data item update associated with the directory entry is a valid dataitem update.
 39. A valid data item update retrieval and storagesubsystem as defined in claim 38 in which said valid data item updateidentifier module includes:A. a directory entry selector module forenabling said processor to select a directory entry; B. a validitydetermination generation module for enabling said processor to use thedata item identifier flag associated with the data item identifieridentified by the selected directory entry generate a validityindication indicating whether the selected directory entry is associatedwith a valid data item update; C. a data item identifier flagconditioner module for enabling the processor to, in response to thevalidity indication indicating that the selected directory entry isassociated with a valid data item update, condition the data itemidentifier flag associated with the data item identifier identified bythe selected directory entry to said at least one other condition; andD. an iteration control module for controlling processor to process thedirectory entry selector module, said validity determination generationmodule and said data item identifier flag conditioner module through aseries of iterations.
 40. A valid data item update retrieval and storagesubsystem as defined in claim 39 in which said iteration control moduleenables said processor to, when processing the directory entry selectorin each iteration, select the one of the directory entries whichprecedes the directory entry selected during the preceding iteration.41. A valid data item update retrieval and storage subsystem as definedin claim 40 in which said iteration control module enables saidprocessor to, when processing said directory entry selector in a firstiteration, select the last directory entry in the storage mediumdirectory.
 42. A valid data item update retrieval and storage subsystemas defined in claim 39 in which said iteration control module enablessaid processor to terminate iterations after the iteration in which,during the directory entry selection step, the first directory entry inthe storage medium directory is selected.
 43. A valid data item updateretrieval and storage subsystem as defined in claim 37 furthercomprising a data item update storage control module for enabling saidprocessor to control storage of valid data item updates provided by saidvalid data item update transfer control on the target storage medium.44. A valid data item update retrieval and storage subsystem as definedin claim 43 in which said data item update storage control modulefurther enables said processor to control storage of data item updatesprovided by a data item update source on the target storage medium. 45.A valid data item update retrieval and storage subsystem as defined inclaim 44, the target storage medium having associated therewith a targetstorage medium data item identifier flag set comprising a plurality oftarget storage medium data item identifier flags, each associated with adata item identifier, the data item update storage control modulefurther enabling said processor to condition the target storage mediumdata item identifier flags in response to the storage of data itemupdates on the target storage medium.
 46. A valid data item updateretrieval and storage subsystem as defined in claim 45 in which eachtarget storage medium data item identifier flag has having a validcondition and at least one other condition, the data item update storagecontrol module enabling the processor to condition the target storagemedium data item identifier flags associated with data item identifiersfor data item updates received from the data item update source andstored on said target storage medium to the valid condition.
 47. A validdata item update retrieval and storage subsystem as defined in claim 46,the valid data item update retrieval and storage subsystem being used ina system comprising at least one other storage medium having associatedtherewith a data item identifier flag set comprising a plurality of dataitem identifier flags, each associated with a data item identifier, eachdata item identifier flag having a valid condition indicating that theother storage medium has a valid data item update associated with therespective data item identifier flag's associated data item identifierstored thereon, and at least one other condition, the data item updatestorage control module further enabling said processor to condition thedata item identifier flag of the data item identifier flag setassociated with the other storage medium to said other condition foreach target storage medium data item identifier flag which has the validcondition.
 48. A valid data item update retrieval and storage subsystemas defined in claim 47 in which each target storage medium data itemidentifier flag has has a valid condition and at least one othercondition, the data item update storage control module enabling theprocessor to condition the target storage medium data item identifierflags associated with data item identifiers for data item updatesreceived from the data item update source and stored on said targetstorage medium to the valid condition.
 49. A valid data item updateretrieval and storage subsystem as defined in claim 48 in which saiddata item update storage control module further enabling the processorto selectively condition the target storage medium data item identifierflags associated with data item identifiers for data item updatesreceived from the source storage medium to one of the valid condition orthe other condition.
 50. A valid data item update retrieval and storagesubsystem as defined in claim 49, the data item update storage controlmodule enabling the processor to condition the target storage mediumdata item identifier flags associated with data item identifiers forvalid data item updates received from the source storage medium to thevalid condition.
 51. A valid data item update retrieval and storagesubsystem as defined in claim 37, further comprising a data item updatestorage control module for enabling said processor to (i) controlstorage of successive data item updates on said target storage medium,(ii) enable the maintaining a storage medium directory for said targetstorage medium, and (iii) provide an entry for each data item updatestored on said target storage medium including the data item identifier,so that the successive entries in the target storage medium's storagemedium directory identify the successive data item identifiersassociated with successive data item updates stored on the targetstorage medium.
 52. A valid data item update retrieval and storagesubsystem as defined in claim 51 in which each entry of said targetstorage medium's storage medium directory further includes an invalidflag having an invalid condition and at least one other condition, thedata item update storage control module enabling the processor tocondition of the invalid flag.
 53. A valid data item update retrievaland storage subsystem as defined in claim 52, the valid data item updateidentifier and the valid data item update transfer control modulecontrolling said processor during a space reclamation operation in whichvalid data item updates are copied from the source storage medium to thetarget medium, the data item update storage control module enabling saidprocessor to, during the space reclamation operation, maintain anauxiliary data item identifier flag set comprising a plurality ofauxiliary data item identifier flags, each associated with a data itemidentifier, each flag having a plurality of conditions, the data itemupdate storage control module further controlling the processor tocondition of the invalid flag of selected ones of the entries of saidtarget storage medium's storage medium directory in relation to thecondition of the auxiliary data item identifier flag associated with thedata item identifier of the respective entries.
 54. A valid data itemupdate retrieval and storage subsystem as defined in claim 53, in whichsaid data item update storage control further controls storage of dataitem updates provided by a data item update source, each beingassociated with a said data item identifier, on the target storagemedium, the data item update storage control module including:A. anauxiliary data item identifier flag conditioning module for, during asaid space reclamation operation, enabling said processor to conditionthe auxiliary data item identifier flag associated with the data itemidentifier of each data item update provided by said data item updatesource to a predetermined condition; and B. a target storage mediumdirectory entry generation module for controlling said processor togenerate an entry for the target storage medium's storage mediumdirectory, the target storage medium directory entry generation moduleenabling said processor to condition the entry's invalid flag to aninvalid condition if the data item update stored on the target storagemedium was provided by the valid data item update transfer control andthe auxiliary data item identifier flag associated with the data itemidentifier of the data item update had predetermined condition.
 55. Acontrol subsystem for use in connection with a processor to form a validdata item update retrieval and storage subsystem for copying valid onesof a plurality of data item updates serially recorded on a sourcestorage medium, onto a target storage medium, each data item updatehaving a data item identifier which is one of a set of data itemidentifier values, a set of said valid data item updates for storing ona valid data item update storage medium, the control subsystemcomprising:A. a storage medium directory including a series of directoryentries each identifying, for a corresponding one of the series of dataitem updates recorded on said source storage medium, the data itemidentifier associated with said corresponding one of the series of dataitem updates; B. a data item identifier flag set module for enabling theprocessor to maintain a data item identifier flag set comprising aplurality of data item identifier flags, each associated with a dataitem identifier, each flag having a valid condition indicating that thesource storage medium has a valid data item update associated with therespective data item identifier flag's associated data item identifierstored thereon, and at least one other condition; C. a valid data itemupdate identifier use the storage medium directory and said data itemidentifier flag set to identify a set of valid data item updates on saidsource storage medium, the valid data item update identifier scanningthe directory entries in said directory in reverse in reverse order and,for each directory entry, determining whether the data item identifierflag associated with the data item identifier contained in the directoryentry indicates that the source cartridge has a valid data item updateassociated with the respective flag's associated data item identifierstored thereon and, if so, determining that the data item updateassociated with the directory entry is a valid data item update; D. avalid data item update transfer control for enabling the processor toprovide valid ones of the data item updates as identified by said validdata item update identifier from the source storage medium for storageon the target storage medium.
 56. A control subsystem as defined inclaim 55 in which the valid data item update identifier module furtherenables the processor to condition the one of the data item identifierflags associated with the data item identifier to said at least oneother condition when it determines that the data item update associatedwith the directory entry is a valid data item update.
 57. A controlsubsystem as defined in claim 56 in which said valid data item updateidentifier module includes:A. a directory entry selector module forenabling said processor to select a directory entry; B. a validitydetermination generation module for enabling said processor to use thedata item identifier flag associated with the data item identifieridentified by the selected directory entry generate a validityindication indicating whether the selected directory entry is associatedwith a valid data item update; C. a data item identifier flagconditioner module for enabling the processor to, in response to thevalidity indication indicating that the selected directory entry isassociated with a valid data item update, condition the data itemidentifier flag associated with the data item identifier identified bythe selected directory entry to said at least one other condition; andD. an iteration control module for controlling processor to process thedirectory entry selector module, said validity determination generationmodule and said data item identifier flag conditioner module through aseries of iterations.
 58. A control subsystem as defined in claim 57 inwhich said iteration control module enables said processor to, whenprocessing the directory entry selector in each iteration, select theone of the directory entries which precedes the directory entry selectedduring the preceding iteration.
 59. A control subsystem as defined inclaim 58 in which said iteration control module enables said processorto, when processing said directory entry selector in a first iteration,select the last directory entry in the storage medium directory.
 60. Acontrol subsystem as defined in claim 58 in which said iteration controlmodule enables said processor to terminate iterations after theiteration in which, during the directory entry selection step, the firstdirectory entry in the storage medium directory is selected.
 61. Acontrol subsystem as defined in claim 55 further comprising a data itemupdate storage control module for enabling said processor to controlstorage of valid data item updates provided by said valid data itemupdate transfer control on the target storage medium.
 62. A controlsubsystem as defined in claim 61 in which said data item update storagecontrol module further enables said processor to control storage of dataitem updates provided by a data item update source on the target storagemedium.
 63. A control subsystem as defined in claim 62, the targetstorage medium having associated therewith a target storage medium dataitem identifier flag set comprising a plurality of target storage mediumdata item identifier flags, each associated with a data item identifier,the data item update storage control module further enabling saidprocessor to condition the target storage medium data item identifierflags in response to the storage of data item updates on the targetstorage medium.
 64. A control subsystem as defined in claim 63 in whicheach target storage medium data item identifier flag has having a validcondition and at least one other condition, the data item update storagecontrol module enabling the processor to condition the target storagemedium data item identifier flags associated with data item identifiersfor data item updates received from the data item update source andstored on said target storage medium to the valid condition.
 65. Acontrol subsystem as defined in claim 64, the valid data item updateretrieval and storage subsystem being used in a system comprising atleast one other storage medium having associated therewith a data itemidentifier flag set comprising a plurality of data item identifierflags, each associated with a data item identifier, each data itemidentifier flag having a valid condition indicating that the otherstorage medium has a valid data item update associated with therespective data item identifier flag's associated data item identifierstored thereon, and at least one other condition, the data item updatestorage control module further enabling said processor to condition thedata item identifier flag of the data item identifier flag setassociated with the other storage medium to said other condition foreach target storage medium data item identifier flag which has the validcondition.
 66. A control subsystem as defined in claim 65 in which eachtarget storage medium data item identifier flag has has a validcondition and at least one other condition, the data item update storagecontrol module enabling the processor to condition the target storagemedium data item identifier flags associated with data item identifiersfor data item updates received from the data item update source andstored on said target storage medium to the valid condition.
 67. Acontrol subsystem as defined in claim 66 in which said data item updatestorage control module further enabling the processor to selectivelycondition the target storage medium data item identifier flagsassociated with data item identifiers for data item updates receivedfrom the source storage medium to one of the valid condition or theother condition.
 68. A control subsystem as defined in claim 67, thedata item update storage control module enabling the processor tocondition the target storage medium data item identifier flagsassociated with data item identifiers for valid data item updatesreceived from the source storage medium to the valid condition.
 69. Acontrol subsystem as defined in claim 65, further comprising a data itemupdate storage control module for enabling said processor to (i) controlstorage of successive data item updates on said target storage medium,(ii) enable the maintaining a storage medium directory for said targetstorage medium, and (iii) provide an entry for each data item updatestored on said target storage medium including the data item identifier,so that the successive entries in the target storage medium's storagemedium directory identify the successive data item identifiersassociated with successive data item updates stored on the targetstorage medium.
 70. A control subsystem as defined in claim 69 in whicheach entry of said target storage medium's storage medium directoryfurther includes an invalid flag having an invalid condition and atleast one other condition, the data item update storage control moduleenabling the processor to condition of the invalid flag.
 71. A controlsubsystem as defined in claim 70, the valid data item update identifierand the valid data item update transfer control module controlling saidprocessor during a space reclamation operation in which valid data itemupdates are copied from the source storage medium to the target medium,the data item update storage control module enabling said processor to,during the space reclamation operation, maintain an auxiliary data itemidentifier flag set comprising a plurality of auxiliary data itemidentifier flags, each associated with a data item identifier, each flaghaving a plurality of conditions, the data item update storage controlmodule further controlling the processor to condition of the invalidflag of selected ones of the entries of said target storage medium'sstorage medium directory in relation to the condition of the auxiliarydata item identifier flag associated with the data item identifier ofthe respective entries.
 72. A control subsystem as defined in claim 71,in which said data item update storage control further controls storageof data item updates provided by a data item update source, each beingassociated with a said data item identifier, on the target storagemedium, the data item update storage control module including:A. anauxiliary data item identifier flag conditioning module for, during asaid space reclamation operation, enabling said processor to conditionthe auxiliary data item identifier flag associated with the data itemidentifier of each data item update provided by said data item updatesource to a predetermined condition; and B. a target storage mediumdirectory entry generation module for controlling said processor togenerate an entry for the target storage medium's storage mediumdirectory, the target storage medium directory entry generation moduleenabling said processor to condition the entry's invalid flag to aninvalid condition if the data item update stored on the target storagemedium was provided by the valid data item update transfer control andthe auxiliary data item identifier flag associated with the data itemidentifier of the data item update had predetermined condition.